London was a very important city both during the Roman period and also at the beginning of the Anglo-Saxon period (with a small lapse in the middle). It had the largest basilica north of the alps which indicates that it held a rather important status, and Tacitus mentioned that it was 'crowded full of merchants and merchandise'. Rome spent a lot of money creating the port and bridge in London as Britain at least in the early few centuries was quite important as evidenced by the fact that until the collapse of the Roman empire, it stationed about 1/8 of the Imperial army in Britain.

Part of the reason for the decline of London was a fire somewhere between 125-130AD and a subsequent decline in population somewhere between 155-160AD. Parts of London were also deliberately demolished- the Basilica was one casualty, the baths were another. There's a number of factors that have been suggested for the overall decline - a plague in between 443-455AD that went around the Roman empire, there was a substantial loss of Roman coin in Britain which shows itself in the reduction in the quality of living quarters and the change in what rooms were used for in the houses.

Most of it was buried, some of it deliberately, some of it by neglect and decay, and with no-one to maintain them, they were often turned into building material for other ventures in later centuries.

The quick answer, I suppose, is that there are Roman ruins in London, just not on the scale that there are in Rome itself. For example, the Roman Amphitheatre which is beneath the Guildhall in the modern City of London, various sections of wall and part of the Forum in the basement of, I believe, a hairdressers, among many others.

The important point to consider is that both Rome and London have been continually inhabited since Roman times and what this means is that what survives tends to be buried underneath the modern cities. The better question to ask, in other words, is why does more remain standing in Rome and so little in London?

A lot of the structures that remain standing in Rome do so because they stayed in use (although not necessarily the same kind of use) in later times, for instance, the pantheon became the church of St. Mary of the Martyrs. Another common occurrence is for parts of Roman structures to be incorporated into later buildings.

The most common fate for buildings of any grandeur was to be quarried for stone to be used in later buildings. An example of this is a stone from Hadrian's Wall found in Lanercost Priory with an inscription referring to one of the Roman legions (The 6th Legoin Victrix) involved in the wall's construction. This happened just as much in Rome as elsewhere, so it wasn't that later Britons had any particular lack of interest in the preservation of Roman structures nor that there was necessarily any specific effort to take Roman structures down.

It's really important to bear in mind that what is now an ancient structure to us was once just a functional building and would have been treated with no more reverence than a modern office block is today - if the purpose it serves is no longer useful to the people living around it, it will be either turned into something that is useful or used to build something useful. The idea that we have that old buildings should be preserved purely because they can tell us things about the period to which they belong is a very recent concept indeed.

So, it's actually remarkable is that so much survives in such good condition in Rome - even though what survives is actually very little in terms of what there was. Most of what survives in Rome does so in the same way as the ruins of London - underground - and there's more of it because Rome was much bigger to begin with. Londinium may have been the largest settlement in Roman Britain but Britain itself was a minor province, meaning that Londinium was never anywhere near Rome in terms of size or grandeur. Very little still stands because very little managed to either remain in constant use or was sizable enough to withstand later quarrying.

There've been plenty of posts on the importance and architecture of Londinium but nobody has yet addressed one very important factor: Mussolini. While the fascist party was in power, Rome was redesigned completely, and much of the impressive monumental architecture (especially the Forum Romanum) was given a much more prominent place in the city.

For example, the Forum had been paved over by later developments, and many buildings had been added to the space in the Medieval and Baroque periods. Mussolini ordered all those removed, so that only the 'original' Roman remains were left. Because the Roman Empire was an important model for Mussolini's ideal state, they were thus used as a way of reinforcing fascist ideology. Another great example of this is the Res Gestae of Augustus: Mussolini had the inscription rehoused to a more prominent place (the building has since been demolished) and even wrote his own - a bizarre piece of work flanked by angels carrying machine guns - to reinforce the connection between them.

So answering your question in a slightly different way: it's not only that Londinium just didn't really have the same scale of monumental architecture in the first place, but also that Rome's ruins served an important ideological purpose during the fascist regime.

Source: Painter, B. Mussolini's Rome: Rebuilding the eternal city, Palgrave Macmillan (2005).

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