Other mistake: In the scene at the end where Farrier gazes upon his burning Spitfire on the beach, the propeller appears to be supported at the end of a simple rod. In fact the propeller would have been attached to a rather solid engine. There's also no internal structure - the entire spitfire was reduced to ashes which was impossible as this was a metal aircraft. Unlike the Wellington, Mosquito or Hurricane which were partly timber and canvas.
Other mistake: When the pilot of the Spitfire is shown ditching into the water, his engine is windmilling at high RPM as he impacts the water. This would have resulted in all of the prop blade tips being bent backwards; however, as it shows him trying to escape the sinking airplane, the prop blades are perfectly straight.
Other mistake: In a scene where the commanding marine officer stands on the mole in a close-up shot, black smoke is rising from sky out of nowhere. Looks like the FX guys forgot to delete the particle effect after deciding to not show a boat with its exhausts in the background.
Other mistake: When Tommy walks down the stairs on the boat to be bombed, one of the soldiers in the top middle of the left side of the screen can be seen pretending to eat a sandwich by repeatedly snapping his teeth near the edge of the crust. (00:31:15)
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.