From that trailer it does seem to suggest that they are correcting the myth the RAF were not around to stop the German planes.
A recent documentary on Channel Four has shown that the RAF and in-particular the Spitfire actually made certain that the evacuation was possible. By stopping most of the German planes getting through. It's just that the troops on the ground could not see the planes fighting at the high altitudes or were inland breaking up the bomber formations before they reached the beaches and the ships rescuing them.
If the film does show this and sticks largely to historical facts in the 2 or so hours, then it will be a good film and for that reason will deliver all the excitement you need.
Education for anyone aged 12 to 16 has made a mess of the world!