I remember reading Edward Gibbon's The Decline and Fall of the Roman Empire as a young man, and thinking how, if only someone had installed a telephone exchange on the Capitoline Hill — with four extensions in, say, Egypt, Greece, Germany and Britain — the whole decadent horror show would've gone under in a matter of weeks, rather than the three long centuries of sacking and re-sacking it eventually took for SPQR to be spiked. Anyway, we know what the decline and fall of a mighty empire should look like: the barbarians hard at it, reducing the noble public buildings to dung heaps; ploughing the fields with salt; desecrating the images of the gods; then raping all the Vestal Virgins and the temple prostitutes before rounding up the vanquished leaders and crucifying them all along the Appian Way, or the A5. Whichever is closer to hand.

But our great and glorious British Empire, which once — must I remind you? — governed one-sixth of the globe with its red and inky rule, declined then fell with hardly any of these humiliations. Why? Because the barbarians who've sacked our mighty capital city did so with commercial rather than martial weapons: our Rome-that-was has been rendered the same as every other clone capital in this wipe-clean world. The Roman patricians were hauled away in chains by foreigners; our ruling class are shackled by multinational chain stores. It's a different sort of humbling but nonetheless a comprehensive one, the beginning of the end of which was marked a lustrum ago (that's five years, for those of you not up on Roman calendrical terminology), when M&M's World opened for business in Leicester Square.

Yes, yes — of course it was a terrible blow that a 35,000sq ft outlet retailing these poxy little chocolate blebs should dominate one of our capital's finest piazzas — but worse, far worse, was to come. Appropriately enough, I learned of the final collapse of the British Empire by telephone: the editor of this august journal rang me, and in trembling tones asked, was I aware that M&M's World was now one of London's top visitor attractions? Well, I told the editor, I was indeed aware of M&M's World itself; how could I not be, when it crouches on the northwest corner of the square, like some brightly coloured and still more infantile version of the Death Star? For these past five years, I've been compelled to make a wide detour when crossing the West End on foot, simply in order to avoid this vast and glassy bit of barbarism. But as to this latest intelligence, no, I obviously didn't know M&M's World was among the most-visited "attractions" in London — more popular than many a palace, museum, park or architectural gem — because had I, I should've immediately done as any defeated and noble patrician should, and ordered a marmoreal bath of warmish water to be drawn, then upon entering it opened my veins.

But before I could take the Roman way out, my editorial liege ordered me to undertake one final mission, arguably a suicidal one, anyway: I was to penetrate M&M's World and report back on what I saw there. So it was, armed only with a credit card and accompanied by three of my children (enlisted to taste the nauseating merchandise), that I entered these once noble precincts, and avoiding the busking beat-boxers in their clouds of spit, and the marauding phalanxes of Benelux exchange students, made my way towards this giant and cosmic solecism.

It's the smell that hits you first: the sickly reek of milk chocolate, peanuts, sugar, skim milk, cocoa butter, lactose, milk fat, soy lecithin, salt, artificial flavouring, corn starch, dextrin colouring and acacia gum. It's an awful lot of ingredients to cram into a little pellet sealed with hardened sugar syrup weighing under a gramme, but it certainly explains the well-known (and synecdochical) saying: "I shall show you the world in an M&M in M&M's World." No, really, who'd have guessed? M&M's World positively honks of M&M's and to enter between its glassy doors and pass under the arch carved in a life-size model of a London bus with giant M&M hubcaps is to feel as I did when I was a small child, when, having finished my tube of Smarties in about four seconds flat, I shoved the waxed cardboard tube they came in up my nostril, and sniffed as deeply as a Studio 54 wild child hoovering up a fat line of Peruvian flake. Yes! It's the smell that hit me first, releasing these deeply buried memories of my tooth-rotting childhood, and with them more festering resentment! For what is an M&M, really, but a Smartie with global reach?

Poor Smarties! Described in their Wikipedia entry as "oblate spheroids", these candy-coated chocolates have none of the world-girdling swagger of the contemporary M&M and yet, it could all have been so different. Forrest Mars Sr, eponymous scion of the confectionery brand, first got (some might say "nicked") the idea for M&M's when, while travelling in Europe with a member of the Rowntree family, he saw soldiers during the Spanish Civil War munching on lentil-shaped, candy-coated chocolates. He hied himself home and together with Bruce Murrie, the heir to the Hershey's chocolate fortune, "created" the M&M (Mars and Murrie, get it?). So, M&M's are a product of confectioners to the gingerbread manor born. (The Rowntree company would go on separately to create their proper and imperial British sweetmeat, the Smartie.) The pillaging gets worse, when we consider the well-known slogan, "Melts in your mouth, not in your hands." If I came to an awareness of intoxication with a Smartie tube rammed up my nose, my sexual awakening was occasioned by a Peanut Treet slowly going gooey on my pubescent tongue, while these words wrapped themselves around the sails of the windmills of my mind.

Yes, the late Sixties were a heady time of sexual experimentation, when even a chocolate-coated peanut could be seen as, well, a clitoris. Because if I remember the peanut Treets slug-line well, I recall still better Mr Robinson, the deputy headmaster, during our sex education class, rocking back and forth on his heels in front of a class of 35 adolescent boys reeking of unexpended spunk, and, his hands deep in his trouser pockets, proclaiming: "I know you may find this shocking, but I can assure you that soon enough you'll really enjoy kissing women's genitals…" How right he was! Although in my case it couldn't be soon enough: I had to wait some years, being tormented by that slug-line, before I had the opportunity to experience anything that appetising melting in my mouth, rather than my hand. It gets worse, though, because what with mergers and acquisitions, the lubricious slug-line got transferred to those hatefully monotone, oblate spheroid candies known as Minstrels, while in America, it became attached to the still more loathsome M&M.

If I bang on at such length about my remembrance of sweets past, it's because I suspect the entire M&M's World phenomenon is just another example of the way our current demographic reversal is affecting every aspect of our culture and society. Once under the fake-bus arch, my crack team of investigators fanned out: I sent the kids to assay "the world's biggest candy wall", namely, a rack of giant optics containing M&M's of every conceivable hue. Meanwhile, I undertook a series of vox pop interviews, stopping the happy shoppers and asking them, what the fuck they were doing there! Obviously, not straight out. Rather, I approached the matter obliquely: were they visitors to London? Where else had they been? Why had they decided to come to M&M's World? And then, the clincher: were they aware that M&M's World was now one of London's premier visitor attractions? I spoke to Paolo and Sofia from actual Rome: they claimed that M&M's were overpriced in their hometown and available only in individual packets, so they were undertaking a sort of "M&M's cruise" (just as Norwegians, apparently, hotfoot it to Sweden to buy the choccies which are overpriced in their neck of the fjord). Fair enough, but others I spoke to — Ben and Laura from Newport Pagnell, the Pesoa family from Oporto, Steve and Milly from Arkansas — had the feeblest excuse for being present: they'd simply seen the much-vaunted "Four Levels of Fun" sign as they wandered the West End, and so wandered in.

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Really, I thought, this makes M&M's World simply a scaled-up version of placing the bonbons by the till. Under such circumstances, flogging the things becomes an opportunistic crime. A crime my children were being the willing victims of: there they were, over by the great wall of tooth rot, plastic beakers in hand, milking the giant optics for their bad mothering lode. But it's baby-boomers such as myself that have really powered the confectionery revolution. Why, should I wish to replicate the Smartie tube-insufflation of my childhood, there are at least four "retro" emporiums within five minutes' walk of M&M's World where I can buy every superannuated Seventies sweet I could possibly desire, from Barratt Sherbet Fountains to Walnut Whips, and back again. As it is with every other aspect of our culture, so the big bulging bellies of us baby-boomers take up all the space, sucking down all possible future innovations into sickly and permanent Now.

Dark green, green and electric green are the different green shades of M&M's available at M&M's World, but why no glaucous or viridian? I dare say the walk-ups who buy "Keep Calm and Love M&M's" T-shirts, or knee-socks patterned with M&M's "clocks", find nothing to disturb them in these huge syringes, but for me a green M&M resembles nothing so much as a temazepam sleeping pill. Known as "jellies" among street addicts, these sedative caplets are not only shaped like the milk chocolate-centred sweeties but also have a sheen suggestive of candy-coating. My youngest, who's 15, claims that if you leave an M&M poised on your tongue for a few seconds, the chocolate melts inside, so that when you bite down on the candy coating you receive a pleasingly piquant gush; but this also reminds me of jellies, which also have a liquid centre, albeit one that blocks veins rather than rots teeth. Yes, yes, I know, hardly the sort of reaction you'd expect anyone to have to "The Sweetest Wall of All", but it gets worse.

In the far corner of the basement floor of M&M's World, I found the counter where, for a price, you can have your M&M's personalised. I swaggered up to the two assistants manning it and asked straight out: "Can I have my M&M's personalised with the words 'Butt Munch'?" And quick as a knife one of them snapped back, "No," and then expatiated further. "We aren't allowed to put anything obscene, political or religious on an M&M." I stood, shocked by this intelligence; on the tip of my sour tongue the observation that any jihadist worth his virgins would probably be considering a rather more spectacular course of action than having a peanut M&M inscribed with "God is Great".

Then her colleague relented a little: "You see, it's all done by this computer," he explained, "and it's programmed to refuse those sorts of requests. You might get away with 'but munch' with one T, though." Might indeed, but "but munch" wasn't what I was after at all. Ever since managing to get a cereal spoon engraved with "Butt Munch" as part of a free Kellogg's "personalised spoon" promotion, I've been on the lookout for more of these opportunities. The French Situationists, a Sixties groupuscule of art revolutionaries, coined the term "détournement" (literally "detouring" or "hijacking") to describe their practise of bowdlerising advertising posters. But my attack on late capitalism takes this subtler form.

"How about…" I chose my words carefully, "'Jeremy Corbyn'. Can I have some M&M's blazoned with the name of the Labour leader?"

The M&M's personalisers thought for a moment, then the female of the species replied: "It's a bit long to fit on an M&M."

I scratched my head then said, "I suppose 'socialism' is out of the question?" It was, as was "liberalism", and when I proposed having a batch of M&M's personalised with "Tim Farron", they just looked at me pityingly.

But really, this is all mere chocolate casuistry, for what is M&M's World itself, if not a political statement of the most stentorian kind? Its London-bus arch, its plethora of Union Jack-branded merchandise and, most especially, its recreation of that seminal-cerebral moment when Messrs Lennon, McCartney, Harrison and Starr walked across the zebra crossing on Abbey Road — a simulacrum which involves life-size M&M's dummy figures substituting for the greatest popular beat combo the world has ever seen. This latter atrocity exhibition is accessed down a spiralling staircase painted oesophagus-red — presumably so punters see the M&M's dummies simultaneously as The Beatles and as giant candy-coated chocolate drops, before they pivot, raise their phones and take that all-important selfie. We're familiar with the notion of a utopia (that's what Jeremy Corbyn is beckoning us towards in his jaunty cap) but perhaps we should consider the notion of the uchronic quite as much as the utopic: for it's this which informs the merchandising at M&M's World, and arguably the political credo of our wider, post-Brexit-vote world as well.

The London bus, the sweetie Beatles, the Union Jacks repurposed to flog T-shirts, the bowdlerisations of World War II public information posters… all of these are intended to evoke a time which never existed, quite as much as a place not yet summoned into being. Yes, M&M's World seems to say — stridently, biliously — there was an era when we all swung together, stuck together, worked together and rotted our teeth en suite. It was an age when brave Brit battlers took the fight to the Hun, then returned home to their thatched castles for a refreshing handful of non-melting chocolate candies. There, surrounded by their ancestral M&M's merch' — plastic bottles full of M&M's, star-shaped plastic boxes full of M&M's, bright yellow cushions in the shape of anthropomorphic M&M's, M&M's tote bags and teapots, M&M's dispensers shaped like outsize plastic cruets — they took their ease, content that all was for the best in a world which you could personalise your M&M's with the family coat of arms. To my way of thinking, while some aspects of this cowardly and costly world have been around for decades — remember Tony Blair's odious "Cool Britannia" — it didn't finally shiver into full and nauseating being until the London Olympics.

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Not that I was actually in attendance (I fled to the States to avoid the whole shit-storm), but I've seen the videos of electric guitar-playing NHS employees riding double-deckers driven by Boadicea, or whatever the ghastly gallimaufry of an opening ceremony it was that Danny Boyle dreamt up. Far from being some triumphant recrudescence of immemorial British values, what it looked like to me was the full and final merge-and-purge between patriotism and commercialism. Henceforth, there would be no Great Britain as was, but only that hateful multinational corporation, so beloved of cabinet ministers (and their tax-avoiding chums) "UK plc".

There's actually a good deal of dead space in M&M's World, considerable nooks and roomy crannies into which a great deal more merch' could be crammed. I saw an M&M's wipe-clean plastic rain mac — but why not an M&M's Crombie, with a bright yellow velvet collar? As for the "Four Levels of Fun", in truth the fourth is little more than a mezzanine; but what I'm chewing over here, is that in almost all of our formerly green and pleasant land, it's merchandising which has taken the place of England's immemorial dreaming.

There was a time when Mars Inc, which manufactures M&M's, experimented with all manner of flavours just as they do with many different colours. They tried candy corn and dulce de leche, cinnamon and chilli nut, but on balance it seems that the punters prefer the gooey consistencies of milk chocolate and the predictable crunch of peanuts. My head in a candy-coated whirl, I re-encountered my offspring in the checkout queue, where, as we snaked towards the tills, we had further retail opportunities. The sales assistant weighed our beakers of sweets and I heaved the plastic: 33 bloody pounds and 96 fucking pence! And that was after I'd sent the oldest boy back to get a smaller pot!

Standing there, I thought back to my childhood, when mighty tribunes such as Harold Wilson and Edward Heath bestrode the world, my brother and I used to nick the loose change my father left overnight on top of his chest of drawers. Then we'd tiptoe out of the house at around 7am, scamper up to the sweet shop on the East End Road and buy Black Jacks and Fruit Salads, four for an old penny, before scampering back home, sugared to the gills, and ready to eat some still more sugary breakfast cereal. Did we envision then a future in which Britain would become a sort of Airstrip 1, upon which planeload upon planeload of colourful button-shaped chocolates would be deposited?

Well, yes, I rather suspect we did, for we loved all things American, and even dubbed the little housing estate we used to walk through on our way to the sweetshop "America", because of its modern, ranch-style houses. I spent a year at grade school in upstate New York when I was a kid, but while I recall little by way of orthodox education, it was a university of candy, what with all those Hershey bars, Baby Ruths, root beers and of course, plenty of M&M's. Once bitten, forever smitten. I suppose what I'm trying to nattily burp up here, by way of answering the troubling question — why is it that M&M's World is now one of London's premier visitor attractions? — is that well-worn, and distinctly un-Roman cri de coeur: We're all to blame! Or at least I am, to the tune of £33.96.