Denham is definitely closest and easiest to get to from Central London via train (from Marylebone)... it's probably about 1 1/2 mile walk from the station to the airfield, but for a young fellow like you this shouldn't be a problem!
Alternatively, my own airfield at Wycombe can also be reached by train, but it's too far to walk from the station and you'd have to get a taxi.
Andy
