Actually, there is a _ton_ of London that is underground, it just happens to not be for pedestrians or the general public. You should take a look at some of th

Actually, there is a _ton_ of London that is underground, it just happens to not be for pedestrians or the general public. You should take a look at some of the planning and tight clearances that are required for things like adding additional cooling/ventilation to the existing tube network. When they were adding a new exit to Victoria as a part of the changes to the station I remember reading about trying to navigate the digging of multi-meter diameter holes with clearances to other underground structures or voids that were 10cm or less.