Here We Go Round the Mulberry Bush (1968, Barry Evans, Judy Geeson)

Filmed in the (then) new town of Stevenage, and has frequent glimpses of London Country area AEC RTs in lincoln green livery..
...but there's also a bizarre dream sequence
Evans's character Jamie and his pal Spike (Christopher Timothy) get jobs as bus conductors;
Only one of these RTs is identifiable, in the wildly-coloured closing credits - it's RT2508 (KXW137):
(thanks Steve Way)