Without any Western Front at all, the Soviets would most likely have eventually prevailed and come to rule the entire continent of Europe. Britain's support for the resistance and governments-in-exile of the various European democracies also effectively prevented a power vacuum that Stalin would have exploited.

Churchill was far more distrustful of Stalin than FDR was. Even if the US sided with the USSR, they would have likely conceded to Stalin more of his goals. Also, without French, Belgian, Dutch, etc. resistance movements and governments-in-exile surviving under British support, the US would have an extremely limited capacity to occupy and rebuild continental Europe, and very little motivation to. Without the Free French movement and Charles de Gaulle, the only seemingly legitimate French leadership after the war would have been the very same government that collaborated with the Nazis, and the Soviets would have just summarily killed all of them and replaced them with their own collaborators.

It's also not entirely clear to me that the US would have joined the war against Germany at all without Britain already being in it. If Britain sued for peace with Germany, they would be unhindered in protecting their empire against Japanese aggression, while the Germans would be unlikely to cooperate with the Japanese against the British. Without Japan attacking either Britain or Russia, there's no reason for a Japanese attack against the United States to provoke American intervention against Germany. Britain's continued belligerency and the escalating American efforts to aid Britain without formally joining the war were necessary parts of joining the two wars into one.

Speaking of which, it also shouldn't be underestimated how deftly Churchill was able to convince the Americans--who had just been attacked by the Japanese, not the Germans--that Germany, rather than Japan, should be their first war priority.