During WWII England utilized an underground bunker not far from Parliament to conduct the war. The high command of the British military planed for the defense of Britain and also campaigns around the globe. Many also had living quarters there as they would spend days underground. After the war it was sealed up. In 1984 it was reopened to the public and is now a museum and part of the Imperial War Museum.. Most of what you see is how it was left in 1945.

Walking the war rooms you can see maps showing allied and axis positions and ship locations from around the world. You can also see Churchill's sleeping quarters as well those of other high ranking people. Conditions must have been bad down there as some people opted to sleep above even during the bombing of London.

I would suggest you buy your tickets ahead of time as you will get right in. The lines for those purchasing tickets there can be long. It is on Horse Guards Road not far from Parliament and is worth seeing.