They didn’t stand 400,000 men on an open beach. The whole thing lasted over a week, mostly they were hiding in buildings and stuff, the lines were just as many folks as they figured they could get loaded in a couple of hours. As for air and ship protection that was England playing the long game and not wanting to lose ships and planes they would need later. As for overcrowded ships being sunk, yeah that’s what happened in the early parts of the war when the Wolf Pack owned the seas.
Not what this new version depicted. Just men on then beach with no cover from elements or as targets. Maybe 6 planes in the air. Most of them shot down. 1 managed to land and set fire to his aircraft and walk away. Not a really good movie of the actual events.