See what you mean; I never realised they did that, although having recently seen 'Reunion at Fairborough', in the sequences filmed at Fairoaks there are some Nissen huts behind what is actually London Transport Flying Club which were erected in what is now undergrowth purely for the film.
As I was working at Farnborough when Hanover Street was filmed, I was no longer living at my parents' place in Chesham hence I was unaware of the B25s at Bovingdon until it was mentioned by my dear old ATC Warrant Officer at Chesham Squadron. He had worked at Bovingdon for Shorts who provided handling services for the RAF aircraft until the day it was 'closed' by the RAF.
There was another short sequence filmed there in the early '70s involving a replica of a Mizar Aircar (Ford Pinto with wings) for 'The Man with the Golden Gun'.