Planewise, I went to the Hendon site a couple of years ago - excellent. there's something a bit wrong about admiring ME109s and Stukas but they do look rather good. The Duxford site is supposed to be very good too and they do a lot of air displays in the warmer months.Here you go siffert_fan RAF Museum in London & RAF Museum Cosford - free family fun activities
I've never been so can't comment on how good/bad they are.
Fleet Air Arm Museum in Somerset is good too if you're down that way.
Naval Aviation Museum, Royal Navy Helicopters, Aircraft, South West