There's the Cabinet War Rooms under the Ministry of Defence, open to the public at certain hours. That was Churchill's main centre of ops. and where he slept etc. but there was also his DOWN STREET dis-used (closed to tube passengers in 1932) Underground Station office-***-hidey hole where War Cabinet meetings sometimes were held. Down Street is a small turning off Piccadilly. Green Park is the nearest open tube station if you want to go there. The street level distinctive tiling and name can still be seen. These days it might be open but I haven't walked around there for a long time. There would be lots of info. on the net, though, Google 'London Transport closed underground stations'.