The IWM at Duxford is a must: it's situated on an authentic Battle of Britain airfield (if you've seen the movie of the same name you will already have seen many of the buildings and airfield) and, as well as being the largest aviation museum in the UK, has a large purpose-built building devoted to American aviation.
https://www.iwm.org.uk/visits/iwm-duxford/whats-here
And fairly easy to get to:
https://www.iwm.org.uk/visits/iwm-duxford/getting-here