I do think the UK would have held out as well if the US had not intervened at all.
In fact, I think Russia was strong enough to defeat the Nazis on her own. They had made a helluva lot of headway before the Western Allies landed in Normandy.
At different points during the war, different countries had a defining impact on proceedings. Had the UK capitulated in 1940, there would have been nowhere for the Western Allies to launch their bombers against Germany. Also, Hitler would have secured the vast resources available in the UK, and would have had no need to garrison over a million soldiers on its Western front - men that could have made a difference in the battle with the USSR.
Next (timeline-wise) would probably come the US, with the great support given to the UK and USSR through lend-lease, and then their actual physical involvement in the war itself. Although obviously in the latter case their hand was forced by the Japanese.
Finally the USSR, who made by far the biggest human sacrifice of them all. Once the Soviet juggernaut started oiling her wheels, there was only going to be one outcome.
And in amongst it all, were the brave people from Poland, France, the Czech Republic, Hungary, etc who done everything they could to fight against the Nazis, whether it was by fleeing to the UK and joining the fight here or taking part in underground resistance movements.
No single nation effected the outcome of WWII, there were many heroes imo.