I was interested to watch the 2017 documentary "Dunkirk: The New Evidence" on SBS last night.
It's pretty good. A couple of things relevant to the movie:
* the town of Dunkirk was a lot more damaged in real life than the movie depicted;
* the RAF was a lot more hated on the ground than even Nolan indicated - there was an interview with a veteran who still seemed to be resentful of them after all these years. Yet the biggest point the documentary made was that the RAF was working hard both over the channel, and far behind enemy lines preventing a lot of German planes getting to the beach; it was just that those stuck on the beach could not see what was going on high and skies and quite some distance from them.
I recommend it. See SBS on Demand.