The Imperial War Museum is brilliant. One of my favourites. Get their as early as you can because it gets busy very quickly. I'm going to be in London in a couple of weeks but I'm skipping the IWM this time in favour of RAF Hendon and the V&A (I want to go see their new photography gallery.)
Tribesman is right - despite what Londoneres will tell you, the Underground is great.
Also, Not military related I know but if you are going to be in London you really should try to visit the British museum. I could bang on about the two Tate galleries and a whole slew of other places but the British museum really is
very impressive.
As for Stonehenge, well, their are other stone circles across the country which may not be as big or famous as Stonehenge but probably retain more of an atmosphere. An neighbour of mine when I was growing up had part of one in her garden. If you get the chance to visit Stonehenge, cool. Personally, I always think of Spinal Tap when I hear the name.
