(1967, Peter Cooke, Dudley Moore, Eleanor Bron)

No, you fool, not the inferior remake with Liz Hurley, in this movie the Devil is played by a smartly-suited Peter Cooke who manages to be both sinister and endearingly loopy at the same time.
Early in the movie a London Country AEC RF passes outside the café where Bron's character is working as a waitress:

Further in, Cooke and Moore board London Transport RTL596 (KGU46), and another sequence has Moore and Bron sitting upstairs on an RT family bus, possibly RTL596 again, but there is no proof:

At the end of the movie a London Transport AEC Routemaster RML is seen in Piccadilly Circus: