In practical terms, the three options given – by air/land/sea – actually boil down to one option: by sea. Escaping Dunkirk by land implies going through the German lines which surrounded the Dunkirk pocket. Perhaps possible at night, but very risky. Escaping Dunkirk by air implies either using an aircraft that’s already on the ground (and I don’t think the BEF had any on the ground at Dunkirk) or landing one in the area. In turn, “landing one in the area” can mean four things. It can mean landing on an actual runway. I don’t think there were any in the Dunkirk pocket. It can mean landing on an improvised runway (like a paved street) using a STOL (short take-off and landing aircraft) like the German Fieseler Storch , the way Hanna Reitsch did in Berlin in late April 1945. I don’t know if the RAF had any such aircraft in its inventory at the time. It can mean trying to land a wheeled aircraft on a sandy beach. I’m pretty sure that would result in a nose-over and crash. Or it can mean landing a seaplane offshore. That option does actually sound practical, but I’m not aware of the British doing it at Dunkirk – perhaps because they had no suitable seaplanes, or perhaps because seaplanes are slow and clumsy flying machines compared to ground-based fighters and thus would have been easy targets for the Luftwaffe.
It should also be noted that the “how would you escape?” concept seems to imply that the hypothetical soldier in question is free to choose what he’d do in that situation. He’s not. He’s operating under orders. Leaving a battlefield on your own initiative in the middle of a battle is called desertion – more specifically, desertion in the face of the enemy – and in principle that’s an offense punishable by summary execution by your own side. Stalin said during WWII that it took a very brave man to be a coward in the Red Army, and the French Army in WWI was said to have shot a number of its own soldiers “pour encourager les autres” (to encourage the others). In WWII, just after the Royal Navy’s final battle with the Bismarck, a British sailor named Joe Brooks who was aboard the Dorsetshire (which was rescuing survivors) saw an armless German sailor in the water; he climbed over the ship’s side to try to get a line around the man, but was unsuccessful in rescuing him; when he got back aboard, the Dorsetshire’s captain threw him in the brig for leaving the ship without authorization.