Thanks for the info there Raindbow Six.
I would say for certain that there are secret underground rail links under London. Probably some of it was just done by reopening/retunnelling and extending existant tunnels that have been closed over the past century or more, as the London underground is vast by any standard.
I know from an ex-girl friend who worked for a county council in southern England that underground operations centres exist beneath many council offices in cities across Britain, as she worked out of one during a drill the local council had. I don't know if they are all nuclear shelters but they are well equipped for emergency situations.
Here are another few links including some more information on Corsham.
http://www.burlingtonbunker.co.uk/
http://www.abovetopsecret.com/forum/thread61322/pg1
http://www.studysupport.info/bunkers.htm
http://www.secretbunker.co.uk/explore_the_bunker.asp
http://www.undergroundkent.co.uk/index.htm
http://website.lineone.net/~civildefence/atom1.htm