First of all, we should remember that following Chamberlains resignation Halifax was actually first in line to be the new PM, it was only when he failed to step forward and take the job that Churchill became PM. Halifax, lest we forget, was in favour of negotiating a peace with Germany at this point.

After that, it actually comes down to the age old question - could the Soviets have defeated Germany on their own? I'm actually one of the few who say no they couldn't. Without Britain in the war, America would not be at war with Germany, and between them both countries supplied huge amounts of materiel to Russia, as well as the fact that a significant chunk of the German army had to be devoted to fighting in Africa, Italy and France.

No UK/US means no support for the resistance in occupied Europe, means less troops required to garrison them, means more troops on the Eastern Front. Furthermore, reading accounts by resistance fighters, early in the war the occupied countries were garrisoned by old men and low quality units. As the resistance intensified veteran units began to perform garrison duties

No UK/US means no threat of invasion. From memory the Germans had 58 divisions in Northern France on D-Day, many of them high quality, veteran units. What difference would just half of those units have made in the East?

No UK/US means no war in N Africa. The Germans lost more men in Tunisia than they lost at Stalingrad.

No UK/US means no bomber offensive. Aside from the disruption this caused to German production of materiel and oil, and the destruction of the Luftwaffe, one cannot underestimate the effect that this had in terms of AAA. 75% of all 88mm guns produced had their barrels pointed at the sky to protect Germany from Allied bombers. There were 40,000 AA guns on the Kahmhuber line. What would those tens of thousands of 88s have done on the Eastern Front shooting at T-34s instead of Lancasters?