The Ladykillers (1955, Alec Guinness, Peter Sellers, Herbert Lom)

"There were a couple of shots of London Trolleybuses and other LT vehicles of the period. The film was shot in the Kings Cross area of London." (thanks Dave Martin)
The rear of a London Transport 2RT2 AEC Regent III RT is seen in King's Cross (screencap courtesy of Neil Fraser):