1. The BEF was only about 200,000 men, which represents less than 20% of the total British forces which invaded France in 1944. In short, Britain still had sufficient manpower to defend itself.
2. The BEF had to leave most of it's equipment behind, so Britain would not have been any worse off materially than they were in reality anyway!
3. The RAF and RN would still be available to defend the UK.
4. The Germans were already overextended at this point, so it's not even sure they would have been able to destroy the British and French.
5. Technically, the Germans DID attack Dunkirk. 800,000 Germans were thrown at 400,000 allies and they died in a ratio of 3:1.
6. The Panzers would have suffered heavy losses in an attack as the landscape was not ideal and they'd be in range of the DN.
So even if they Germans had attacked and won, they would have suffered heavy losses. The only thing Britain would have lost additionally in this scenario is their men, as in the real world they did lose all that heavy equipment anyway. Britain would obviously have been badly mauled by the loss of 200,00 men, but the Germans might have lost a lot of men and equipment too.