Alan Brooke was sent to France after Dunkirk to see if a British presence in France could achieve anything.
Churchill was not finished with the French operation in any way. Brooke was realistic enough to get as much out aqs possible.
That said, the German operation did not achieve its stated goal. Blitzkrieg was having as a goal: Conquer France and neutralise BEF. It was not on the carsd to have them sailing away. They should have been surrounded and made to surrender (or get killed). That was the whole idea behind it. It is stated quite precisely by Deighton in his book: Blitzkrieg.
So, let's imagine it goes according to plan: BEF (not only Dunkirk) is made to surrender, which entails all the later top generals are now POW's (Monty, Alexander, Brooke, etc etc).
UK's army of 350,000 just gone, all materiel gone, top leadership gone, etc.
If Germany would not attempt any Battle of Britain, nor Sea Lion, it would be a stalemate.
Would US support Britain? maybe not. After all Kennedy et al were not impressed with British war efforts.
Would Britain have accepted terms? If Churchill's cabinet should fall, Halifax would have been in (as speculated in other threads). That may have made it possible.
Without US involvement and with a Britain defeated on the continent, I cannot see any major European war effort.
Middle East would have been different if Rommel had started earlier (chuck him another 3-4 divisions), Battle of Atlantic would not have happened if the US didn't support UK.
However, that would all have required a much more strategic or even global world view, which Germany simply did not have at that time (Raeder was the only one trying to present a "global" war plan - much like Brooke had a "global" war plan).
Ivan