When Harry is 'dropped behind the lines' in Poland, the plane they showed had 3 engines, but the RAF didn't have a plane in the 1940 like that. The only aircraft that fits from that era that may have been in the RAF is a Ford Trimotor built in the early 1930s, but that had a range of about 500 miles so a trip to Poland and back would be impossible.
The squadron code on the Spitfires are on the wrong side of the RAF roundel, and none carry an individual callsign. In late mid/late 1940, the Spitfires of 611 Squadron also carried a white band just forward of the tail and red, white and blue vertical stripes on the tail.
There is a scene in an RAF hangar in 1940 with a Spitfire in 1944 D Day markings (i.e. the black & white stripes on the wings).
When entering a hangar Lois walks past a later mark Spitfire not yet in production during the Battle of Britain.