Those Brits were lucky that Hitler was suffering from a delusion that he could talk Great Britain into joining up with him against the Soviet Union. German Generals were horrified that Hitler held them back and kept them from killing and capturing every single retreating Brit... which they could have done.
I think it’s a bit of a myth that somehow Hitler “spared” the BEF, he didn’t, it’s just that he thought they were already beaten and not worthy of too much attention.
The main enemy was the French army and they were at that time still fighting, Hitler was afraid that he might have already pushed his luck so he eased up the advance a bit to let his infantry catch up with his Panzers before striking the final blow against the French.
Goering promised him that his Luftwaffe could finish off what was left of the Brits on the beaches so he left Fat Herman to get on with it, a relatively cost-free option.
Unfortunately like every task the Luftwaffe was given; knock out the BEF, blitz England into defeat, supply the troops at Stalingrad, defend the homeland, they failed miserably.