I thought the reason underground tube lines were less common south of the river
was the number of underground streams?
(Please don't tell me this was an urban myth!)
It might be less of a problem with more modern tunneling techniques and deeper
lines, but the cost would surely not be justified by any useful purpose?
Don't know about you but I prefer riding at roof level - you can look out of
the window, and get some ventilation.
Regarding consideration at the time of World War II, there might have been an
obvious reason then for wanting to put everything underground.