Revealing mistake: The ship set up for the capsize scene is supposedly mid channel and heading for England when it is bombed by the Heinkel, but it is at anchor.
Revealing mistake: In some of the shots where multiple British soldiers are together, most have their rifles (Short Magazine Lee Enfield, No. 1 Mk 3) slung or are holding them. Some of the rifles are dummy, toy rifles that look nothing like the Enfield and are just to stand in.
Answer: That's exactly how it was. If you search for images of the evacuation, the troops were stood in lines, waiting to be evacuated. The Germans were held at bay, and the air raids were periodic, so there was little risk on the beaches.