The Dining Room in the Oil Rig

If there's a building which encapsulates in one structure what happened in Britain in the 1980s, and what afflicts it still, it's Lloyd's of London. Designed by Richard Rogers in 1979 and completed, just to coincide with the City's 'Big Bang' in 1986, it is usually interpreted in one of two completely inadequate ways. For architectural history, it's a monument to 'High-Tech', a style which arose in the mid-70s as a sort of last flicker from the white heat of the technological revolution, at the hands of currently ennobled, often American-trained architects - Baron Foster of Thames Bank, Sir Nicholas Grimshaw, Sir Michael Hopkins, Baron Rogers of Riverside. High-tech, or a version of it, has been the dominant form of architecture in the UK for the last two decades, though you can read a lot from the change in its functions - in the '70s most of the above were designing factories, now, with rare and telling exceptions, they design office blocks, cultural centres and luxury flats with a still residual 'industrial aesthetic'.

The other thing it is known as is a huge metallic embodiment of the Big Bang, a Thatcherite machine for underwriting in (it features on a Five Star sleeve, and a shop in the basement still sells Athena-style framed pictures of it in moody monochrome). Neither of these give any even slight indication of how monstrous, compelling and utterly fucked-up Lloyds is; the architectural critics can't talk about much more than the detailing, the anti-capitalists can't look beyond its (admittedly extremely unpleasant) function. In order to really capture it's weirdness you have to go inside. I've tried to get here for Open House weekend for most of the last decade, finally making it last month, with these people, the latter of whom took some of these photos.

One of the many things Lloyds is about is a strategy of tension between the two complimentary factions of the British ruling class. Before Rogers, the insurers were housed in a neoclassical building built as late as the '50s, contemporary with the Seagram Building - an embodiment of a practically unchanging British gentlemanly capitalism, resistant both to modernism and to swanky, brash American finance capitalism. On one level, Lloyds is Weinerisation to the nth degree. It houses one of the oldest institutions of the City of London, the insurance firm which can date itself to 1688 (neatly contemporary with the 'Glorious Revolution'), and it houses them in the most astonishing futurist structure ever erected in the UK. If it evokes any previously existing buildings of any kind, then they're almost always industrial - oil refineries, or the North Sea Oil Rigs which were built off the east coast of Scotland in the '70s, much beloved of high-tech architects. Both of these are visually striking typologies because of sheer utility, because their functional parts are in no way sheathed or hidden, and because the refining process requires the baffling, twisting intricacies of pipes and gantries. Like so many things with Lloyd's, you can just tick off the political-economic resonances - the oil boom that kept Thatcherism secure in its confrontations with the unions providing inadvertent inspiration for the aesthetic of the City itself at the exact point it was let off the leash.

Maybe this was some kind of unacknowledged appeasing of the gods of industry, paying tribute to it at the same time it was destroyed. It's also possible that Lloyds was and is especially thrilling for people who have never worked in a factory, the only other kind of place where services, pipes and ducts are habitually left so bare, in those places because 'nobody' is looking. Maybe. If there is a specific non-industrial built precedent, though, it's Rogers' earlier Pompidou Centre, the first of a very long and still unbroken line of non-specific cultural centres and tourist draws with wilfully spectacular architecture erected across Western cities. The 'Beaubourg' is often considered to be a '60s dream built, Joan Littlewood and Cedric Price's adaptable, anti-architectural 'Fun Palace' completed and then named after an anti-68 Gaullist. The 68ers immediately moved to disavow it, of course - the text So-Called Utopia of the Centre Beaubourg was the gauchistes' 'don't give me what I want because that's not it' response - but if it looks like a Fun Palace, quacks like a Fun Palace, etc. You can see where I'm going with this, right? An industrial aesthetic is used for FUN and then is used for CAPITAL. The finance-entertainment complex.

What makes visiting Lloyds such a bizarre experience, however, is seeing how the underwriters have conserved so many elements of their atavistic previous existence. These remnants are scattered around the new building, decontextualised fragments ripped from 1763, 1799, 1925 and 1958, rudely riveted onto the ducts and pipes. There's the antiquated uniforms worn by the service staff; the front facade of their 1920s offices is held up like a trophy on street level; inside, the Lutine Bell sits at the foot of 'The Room', more of which later; several paintings and bits of furnishings survive from previous buildings; and strangest of all, a complete 18th century dining room by Robert Adam was preserved and recreated. At first, it seems like these are tokens kept on a sort of reservation of gentlemanly capitalism in order to placate the old guard. After a while you realise that what is really happening here is more like a marriage, a reconciliation, a mockery of Wiener's idea that there should be any difference or hostility between the capitalism of gentlemen and the capitalism of industrialists. That comes about especially forcefully in The Room.

Agata mentions Koyaanisqatsi the first time she sees The Room, and that captures some of its sense of controlled, mechanised mania. It's an enormous, multi-storey concrete atrium dominated visually by two things, on an axis so that the link between the two is unavoidable. There is a web of criss-crossing escalators, which can take the client to the underwriter at speed. These align with the open-plan offices on every side, creating a sort of visual simulation of industrial activity. As you see all this it's hard to imagine that nothing is actually being produced here; the look of some putative industrial hub employed purely for the purposes of immaterial, literally speculative finance. The open floors and the dynamism of the escalators draw the eye straight away to the more sentimental of the assembled, decontextualised objects, the Lutine Bell itself. What you can see is a neoclassical rostrum housing the bell itself, made in the 1920s, mahogany and Corinthian columns, with an antiquated clock on top. The bell inside is rung when a member of the Royal Family dies, and on the rare occasions when a ship they have insured sinks, as was its original function. After that, look up, and you'll see a glass barrel-vaulted roof. You're in a gigantic '80s version of The Crystal Palace, the 1851 iron-and-glass fantasia that Martin Wiener considered British industrial capitalism's unsurpassed zenith. These two sentimental remnants are what they whole high-tech assemblage revolves around. Like the Gothicism of services on the facade, the Room is a quite ridiculously thrilling thing to behold; you have to catch your breath and remind yourself where you are. What this is.

With its glazed lifts, moving parts, girders, cranes, components all crammed into a tight, fierce, metallic mesh, Lloyds has always been a building that has (on me, at least) much the same shivers-down-spine effect as The Human League's 'Dancevision', or 'Strings of Life', or 'Trans-Europe Express': a mechanical sublime that sweeps away any residual humanist resistance with your willing participation. Fully aware of this, there's also a series of get-out-clauses left here by the architect. Rogers was and is a figure of the soft left; as a Labour Party peer, he'll have been one of those who were the NHS' unlikely last line of defence last week. The other stylistic influence here, one which Rogers draws attention to in his books, was the unbuilt projects of the early Soviet Union. The lifts shooting up and down the metal frame are taken from Leningradskaya Pravda; the overwhelming metal-on-metal rush of the street facade is taken from Iakov Chernikhov; the irregular, techno-Gothic approach to the skyscraper is from Ivan Leonidov. So add to the list of ironies the era when the USSR was considered to be capitalism's gravedigger being evoked on the eve of its suicide, for the purposes of the forces that would soon drag its territory into a ferocious gangster capitalism. Another get-out-clause is adaptability. The building is adorned at the top by fragments of the cranes used to construct it, as if to tell us that the thing is in flux; the floors, too, are moveable. The suggestion seems to be that one day it could all be made into something else by someone else. The building is about to be Grade 1 listed, so that's certainly not happening, pending another glorious revolution. Then there's the promise of an organic, reformed and reformist city, which made Rogers a 'New Labour consigliere', in which capacity he was probably the last major British architect to have any ideas about society whatsoever. From 1997 to 2010 the architect had a semi-governmental role advocating street life, compact cities, let's-be-like-Barcelona-or-Berlin-rather-than-fucking-Texas. But the Lloyds Building, no matter how astonishing it might be to look at as a passer-by, meets the street with a moat.

The real moment of madness in Lloyds is the Adam Room. While much of Lloyds evokes the more ruthless side of '80s cinema - a John Carpenter film, The Terminator, Robocop or Gremlins 2 could all be shot here - this place is pure Tarkovsky. It's the last scene of Solaris, where the alien intelligence re-creates the familial hearth. On the 11th floor, the high-tech corridors, with their Gigerish sculptural ceilings, suddenly meet a white concrete block. That concrete block is decorated with classical details. Lloyds is not generally thought to be postmodernism in the usual sense of irony and historical montage - in fact it's often presented instead as 'late modernism', a strident keeping-of-the-faith; Rogers' continuing role as antagonist to the Prince of Wales helps that presentation. Yet here's an absolutely pitch-perfect bit of pomo, a seemingly mocking, parodic reproduction of an Alexander Popish 18th century thrown into a completely alien context.

Walk into it, and you're as far into the heart of the establishment as a commoner is ever likely to get (one weekend, every September). The Adam Room, named after its designer, was originally part of Bowood House in Wiltshire, was commissioned by the first Earl of Sherborne, and is rammed so full of objets d'art that ten increasingly head-bangingly boring series of Antiques Roadshow could be built around Michael Aspel inspecting it piece-by-piece. The sensation it creates is of reaching the inner sanctum of the great parasite itself; all that outside is just for show, a display of how sprightly and modern and with-it we are, a delicate subterfuge, an elaborate joke about deindustrialisation where we can look at paintings of galleons while the shipyards are closed. In here, Lloyd's of London are the same organisation that built itself on the slave trade; it's a time-machine that physically brings Old Corruption back to the site of its inception. They play at modernisation, but always keep this place in reserve, are always able to return to it. We queue to get in outside the Palladian bunker, and then circle round the table for our allotted time.