In the scenario where the vast majority of the BEF is captured or killed, Britain has to be concentrating on rebuilding the army and also dealing with greater than OTL losses of ships and aircraft. While essentially all of the heavier equipment of the BEF was lost on the continent, a fair number of soldiers came back with rifles and personal kit (web gear, canteen, helmet, etc). Yes, many came back with just the clothes on their back but a fair amount of useful light equipment did come back. In addition to replacing the heavy equipment, more ships, and more aircraft, you now have to produce a lot of rifles and mundane but necessary equipment like canteens. It strained Britain, with a good deal of US assistance, to replace all of this lost equipment, every extra set of web gear, rifle, or canteen makes doing this that much more difficult. WE know that a German invasion of England was not going to happen, the British did not know that even if they saw it as improbable. Don't forget that ITTL resources to deal with German troops should they land are even less than OTL and destroyers and other RN ships are fewer as are RAF fighters.
The reason all of this is important is that the UK still has to reinforce Singapore and elsewhere against the Japanese potential threat, even if Italy is not in the war yet they can't ignore North Africa & the Red Sea area. All of this on top of a much larger need to re-equip the rebuilt army at home, probably with less American assistance for reasons that have been listed before. Whether than has been an armistice or Phony War part deux, if a German invasion of Russia goes off in spring, 1941 the UK simply won't have that much "stuff" to give the Russians.
Roosevelt is on thin ice with neutrality acts, providing arms to the UK when the USA needs all that stuff to rebuild its own forces. If the situation in the UK with a disaster at Dunkirk is as described, much weaker militarily, and the Germans and French reach or are seriously negotiating a final agreement, and the Germans offer the British a no harm no foul peace, Roosevelt will have a very difficult time in giving the sort of aid and making changes in neutrality laws that happen OTL. Also the "Western Atlantic Defensive Zone", US ships in convoy escort & so forth. ITTL the UK would be very fortunate to get the same credits and material they did OTL, I expect they would get less with an admonition to cut a deal.
The reason all of this is important is that the UK still has to reinforce Singapore and elsewhere against the Japanese potential threat, even if Italy is not in the war yet they can't ignore North Africa & the Red Sea area. All of this on top of a much larger need to re-equip the rebuilt army at home, probably with less American assistance for reasons that have been listed before. Whether than has been an armistice or Phony War part deux, if a German invasion of Russia goes off in spring, 1941 the UK simply won't have that much "stuff" to give the Russians.
Roosevelt is on thin ice with neutrality acts, providing arms to the UK when the USA needs all that stuff to rebuild its own forces. If the situation in the UK with a disaster at Dunkirk is as described, much weaker militarily, and the Germans and French reach or are seriously negotiating a final agreement, and the Germans offer the British a no harm no foul peace, Roosevelt will have a very difficult time in giving the sort of aid and making changes in neutrality laws that happen OTL. Also the "Western Atlantic Defensive Zone", US ships in convoy escort & so forth. ITTL the UK would be very fortunate to get the same credits and material they did OTL, I expect they would get less with an admonition to cut a deal.