Episode 1.03:
A glimpse of what is believed to be a Neoplan NH22L, also a Saviem SC10.
Episode 1.09:
London Transport RT3771 (NLE878) appears on route 12.
Episode 1.11:
An unidentified London Transport AEC RT.
Episode 1.12:
A Karosa SL11 and a Škoda 706RTO
Episode 1.17:
Features London Transport AEC Routemaster RML2688 (SMK688F)
Episode 2.02:
A Pegaso 5031/Irizar
Episode 2.11:
An AEC Reliance/Plaxton Elite is seen parked at Heathrow Airport.
Episode 2.25:
London Country AEC Routemaster RML2429 (JJD429D) is seen in High Barnet, operating from Garston garage.
Episode 2.22: Shadbolt
"....Has an Edinburgh Corporation Leyland Titan PD3 speeding through the Grassmarket in long shot in the pre-credits sequence - (on route 2) - unidentified." (thanks Scot Fergie)
(Compiled from my own observations and those recorded at the Internet Movie Cars Database)