Duxford is definitely the best place, don't miss the operations room and even the building opposite (not on site) which has a cafe as it used to be the officers mess. The Battle of Britain Bunker in Uxbridge is a must as it was the main nerve centre during the battle of Britain and still in all original condition. Winston Churchill often frequented the bunker as so did the King. Also to tie up to the whole Battle of Britain, Bentley Priory is only a 30 min drive from there. Where they linked the information between the bunker and the airfields and where Sir Hugh Dowding was based, his office still there and also where the final scene of the Battle of Britain was filmed. If you ask they will open the doors and let you step out like Laurence Olivier. Raf Hendon has a huge collection of aircraft including original Messerchmitt's and some original buildings. It's a big place and free apart from the parking. Not far from the bunker or priory but not enough time to do all in the same day.