Actually, that's not true. While you are correct that there are currently two flying Mk.1 Spitfires flying in the world that were shot down and crash landed on French beaches during the time of Dunkirk, neither one was used in the film. There were 2 Mk.1s used for the film (X4650 which appeared as LC/R9612 and AR213 which was marked as LC/R9632). X4650 was delivered to the RAF in October 1940 and AR213 was delivered to the RAF in July 1941 so neither were around for Dunkirk. The third Spitfire used in the movie (EP122) is actually a VB and although the cannon barrels were removed, you can still easily pick it out of the formation because of the cannon bulges on top of the wings.