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.
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.
Fleet Air Arm Museum in Somerset is good too if you're down that way.
Naval Aviation Museum, Royal Navy Helicopters, Aircraft, South West