These photos were taken by Knut Erik Hagen in July 2003, at the Imperial War Museum.
Karen adds: This is the B-type Omnibus of the London General Omnibus Company. These were used as troop transports, ambulances, and rolling pigeon lofts in Flanders and France. This is the B43, aka "Old Bill". Another bus, B340, is at the London Transport Museum, IIRC.