[justincormack]

Lots of countries do where the underground is near the surface, but London it is very deep.

[zulln]

Quick googling gave that Stockholm is deeper on average, although the deepest point of London metro is currently deeper than Stockholm. No drastical differences that should affect this question I think.

(This will change when they are done with Sofia station, a new station in Stockholm that they are building 100 m below the surface.)