Bletchley Park near Milton Keynes - absolutely fascinating to see what they did there, and all with no recognition at the time - I think I found it so fascinating as it is as much about the people as about the war, the technology, etc. Churchill estimated that the people working at Bletchley shortened the war by years because of their code breaking.
Well worth a visit - and it needs funds as the buildings are starting to decay which is scandalous considering what else we pay/fund to upkeep; this place was absolutely key to our winning WWII.