Despite being set in England the American origins of movie are betrayed early on. The arrival of the police car is heralded by a siren at a time when British police cars used bells. Also, the bus which nearly collides with Phyllis and Hugh has the passenger entrance on the right hand side. A British bus would have the entrance on the left hand side due to driving on the left.
In the opening sequence the pillar-box wobbles considerably in response to the explosion. With a real, metal, post-box this would not be possible.