Specifically, I am asking in light of how the Allies used Britain as a massive aircraft carrier and launch point for their bombing runs of mainland Europe, etc.
With Britain unavailable for this purpose, what would the Allies have done? Is there any evidence of a Plan B formed by the UK/USA in anticipation of this probability?
I am sure the US's officially neutral stance (as this would have happened pre-Pearl Harbor) would have changed if their ally Britain had fallen but would the war have basically been ended if the British had yielded to Germany? Did they have any ideas how they were going to retake Europe without the conveniences that using Britain afforded them?
(I do hope I was clear enough)
I checked the FAQ page and didn't see anything.
In the (extremely improbable) event of a successful German invasion in 1940, there were some plans to sail the fleet across the Atlantic, set up a government in exile, and continue the war from Canada.
However, in regards to your question, there was no 'allies' at this time. Britain stood alone with her Commonwealth and Empire from the fall of France to the start of Barbarossa.
Thus the allies did not have a plan because the allies did not exist.