"At the beginning of the film, the Janette Scott and Ian
Carmichael characters have a scene aboard what looks like an RTL on the Embankment in London. This involves them bumping into each other at the bus stop. He is running for the bus, she is getting off loaded with parcels and they both end up on the pavement. It looks better than it sounds. There's a very nice long shot of the bus itself." (thanks John Thornley) The bus in question is RTL908 (LLU898).
At the end of the film the Alistair Sim character
boards RT3948 on route 107 to Ponders End,
closely followed by the Terry-Thomas character who boards following RT365 (HLX182) also on route 107 to Ponders End
Obviously number 107 buses travelled in pairs in those days. The two RTs look immaculate and as the film was made at Elstree, it's possible they were loaned out from nearby Aldenham for the shoot, but that's purely
speculation." (thanks again John Thornley).
RT365 at least was fresh from overhaul at the time the scene was filmed. Mike Lloyd also notes: "There are various street scenes in the film and in one a couple of RT-family buses on route 183 can be seen, together with a Green Line RF rear view receding in the background."
This RT can't be definitively identified, but it may be RT3595 (MLL905):