Cadburys Creme Egg
Features a London AEC Routemaster/Park Royal seen from behind. "The bus is RM1913 (ALD 913B) which was one of those recently brought back to the capital by TfL." (thanks Graeme Selway)
Cadburys Dairy Box
Bob Wingrove found this circa 1961 advert featuring an AEC RT:
Cadburys Dairy Milk
Jolly chocolate bar ad presented in the late 1970s by singer Cilla Black shot in Brighton has the rear of a Southdown Daimler Fleetline/Northern Counties in the background; then a Brighton Corporation Leyland Titan PD2 passes:
Cadburys Double Decker Former London Transport AEC RT2275 (LYR709) was the original promotional bus; I assume this is it.... In a more recent version the doubledecker is AEC Routemaster RML2613 (NML613E) also painted purple - the driver is comedian Charlie Chuck: