I think some people are viewing this movie with the wrong expectations. This is a movie about being there, putting you in the position of being there, the fear and tension that you would feel.
It's not about blood guts gore and action. It's not about history in a factual way, it's not about the...
Would depend on the starting height of course. According to an aviation forum a spitfire would lose approx 2000 feet per mile glided. Although to be fair another forum says from approx 6500 feet could glide for 24km.
There wasn't supposed to be any development of the chartacters, that was deliberate. The film was meant to be subjective about the event, not about the characters.