A monument to 'high-tech', the Lloyd's of London building marries the capitalism of gentlemen with that of the industrialists

Richard Rogers's 1986 headquarters for the insurers Lloyd's of London has just been listed Grade I. This makes it, along with the Royal Festival Hall, one of the few 20th-century structures to be placed at the same level as, say, St Paul's. But, like the gothic cathedrals it so closely resembles, Lloyd's was not meant to be an entirely finished product. Look up to the top of its facade, and you'll find cranes are still there, left when construction ended, to make clear it could still be extended up and outwards. The gothic cathedrals did grow in this manner, but then they didn't get preservation orders 25 years after they were built.

There should be no doubt whatsoever that Lloyd's deserves its listing. But for a building so famous, Lloyd's is not well served by writers and historians. It is usually interpreted in one of two completely inadequate ways. For many, it's a metallic embodiment of the Big Bang, a Thatcherite machine for underwriting. In architectural history, it's a monument to "high-tech", a style that arose in the mid 1970s as a sort of last flicker from the white heat of the technological revolution, at the hands of currently ennobled architects – Lord Foster of Thames Bank, Lord Rogers of Riverside. High-tech, or a version of it, has been the dominant form of architecture in the UK for 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 they design office blocks, cultural centres and luxury flats with a still residual "industrial aesthetic", including the world's most expensive One Hyde Park.

Lloyd's captures the tensions between industrialism and the "new economy" of financial services, then tries to resolve them. Before Rogers, the insurers were housed in a neoclassical building built as late as the 50s – an embodiment of a practically unchanging British gentlemanly capitalism. It was meant to reassure, to look eternal. If the 1986 replacement evokes any previously existing buildings of any kind, then they're industrial, almost temporary structures – oil refineries, or the North Sea oil rigs built off the east coast of Scotland in the 70s, much beloved of high-tech architects. These are visually striking 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. The North Sea oil that kept Thatcherism secure in its confrontations with the unions provided inadvertent inspiration for the aesthetic of the City itself at the exact point it was let off the leash.

Lloyd's marks the point in British architecture where industrial features became something to enjoy in and of themselves; not coincidentally the point where industry itself faced forcible decimation. Maybe those bared ducts, those moving parts, those steel surfaces and gigantic, top-lit open spaces for working in were all some kind of unacknowledged appeasing of the gods of industry. It's also possible that Lloyd's was and is especially thrilling for people who have never worked in a factory, the only other kind of place where services are habitually left uncovered, in those places because "nobody" is looking.

What makes Lloyd's such a bizarre place, however, is seeing how the underwriters have conserved so many elements of their atavistic previous existence. These remnants were 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 earlier neoclassical offices is held up like a severed head. Inside, the Lutine Bell sits at the foot of the enormous, multilevel trading floor 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 the notion that there should be any difference or hostility between the capitalism of gentlemen and the capitalism of industrialists.

Inside the Adam Room, Lloyd's of London are still the 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. British capitalism plays at modernisation, but keeps this place in reserve, as its ancestral home. Now, Lloyd's itself will be kept as a time capsule, a structure that can receive only the tiniest changes. When future generations want to know what happened to power in Britain in the 1980s, their questions will be answered here.