Really depends on the circumstance. Politically the UK was probably more vulnerable to a peace deal if things had been handled differently in 1940 or if Churchill wasn't in the picture for some reason in 1940. The USSR could be knocked out too, militarily speaking, if things had been handled differently, but at a much higher cost. I guess you could say that the UK could have been knocked out easier for political reasons and because there was a period where it was fighting alone, rather than the USSR, which was only ever fighting with at least one very critical ally (the UK), but it would require some important PODs.