The Flipside of Dominick Hide (BBC sci-fi drama, 1980)

Part of the BBC's long-running 'Play For Today' strand. The eponymous hero travels back in time to the 1980s to study London's transport system. He is distracted by the lovely Jane (Caroline Langrishe), and we see much more of her than the buses. There are however odd passing Routemasters:
....and aerial views of RMs and DMS-class Leyland B20 Fleetlines, also an AEC Merlin