I've been listening to "We Have Ways of Making you Talk" which is doing an 85th anniversary take on Dunkirk. Talk about the hinge of history. So many variables. Apparently the decision for the Germans to back off the pocket around Dunkirk was driven by Hitler's vanity when confronted by Generals who had through their own initiative had crossings over the canal line which could be exploited to drive on the beach and perhaps capture the entire BEF and French allied forces, and confronted by more cautious generals did the one thing he would do in that circumstance: he exerted his prerogative to exert his prerogatives and issued the halt order.
Meanwhile, Churchill was desperate to prevent Halifax from convincing the war cabinet to sue for peace and he was fortunate to have Chamberlain who had been burned by Hitler and Mussolini on his side.
Without Dunkirk, its a totally different war. Without Britain holding up the Germans in Eastern Europe and Africa and the Battle of Britain, they have much more to throw at the Soviet Union in 1941 (and could have started earlier in the year).