"Alfred Hitchcock film opens with Stewart, Day and son riding a bus in Morocco. Later on, London Transport double deckers are shown outside Albert Hall and other London locations." (thanks Bill Vandervoort).
I can't vouch for the Moroccan bus (sixcyl at the Internet Movie Cars Database has it as a Floriat C14), but other buses seen are a Bedford A tractor and trailer airport bus, a Chausson APH-52, a BEA AEC RF/Park Royal, and two London Transport RT-family buses.