My first guess it that not much would have changed. Perhapst the main impact would be greater Soviet influence in a post-war Europe.

Germany would still have stopped in the West about where she stopped (Vichy). She would still have turned East. She probably still would have been beaten by Russia, although it would have been harder due to a likely reduction in the UK/US aid and the possible absence of a second front.

Japan and US would still have come to blows and the US still triumphed. Given Hitler's folly and US concern over Nazi supremacy in Europe, the European and Pacific wars would still have become united in a World War. Regardless of how it played out, it is hard to see the US + USSR losing to Germany + Japan.

Perhaps the main difference is that US landings in Europe would have been delayed or perhaps even never happened, giving the Soviets more influence. Without the UK as a staging point, it would have been much harder for the US to invade Europe. And without the UK as an ally, she might have been less inclined to try. However, the US did manage to cover large distances in the Pacific War, so landings in North Africa or elsewhere as a staging point are still conceivable. And fear of Nazi or Soviet hegemony in Europe would still have provided a rationale for direct intervention.

Britain would still have lost her Empire. Not fighting a war might have kept her stronger (although this is dubious, at least in a military and moral sense). But there was no way she could hold on to India or Africa, with millions of restless people demanding independence and an anti-colonial US and USSR as superpowers.

I just don't see the UK's role as being decisive in WW2, unlike that of the USSR or USA.