Without US support, I'd strongly suspect that the UK would remain "technically" in the war, but be all but powerless to do anything directly threatening to Germany itself. Sure, it could still beat up the Italians in North Africa, and send a few bombers over Germany at night (because daylight raids would be suicidal), but nothing comparable to the massive daylight bombing missions launched by the Americans, and no hope of driving up through Italy or making a credible landing attempt in the Netherlands, Belgium, or France. Germany would still be woefully unprepared for a Sealion scenario (but play up the possibility in propaganda, which would force the UK to commit forces to the defense), which would leave the two powers at an impasse. In essence, Germany would be free to dedicate a much larger portion of its forces to the East, while it and the UK probed and prodded in relatively ineffective indirect economic warfare: submarine attacks on shipping versus night bombing raids.
How that affects the Soviet front is uncertain: not good for the Soviets, but would it be enough to tip the balance? I have to assume that it would, at the very least, drag out the war by several more years, if not force some form of negotiated truce. Ultimately, I suspect that either (or both) Hitler and/or Stalin would fall to internal dissent and assassination, and that Churchill would be replaced at some point, leading to a very different political situation.
Oh, sorry, I guess this is supposed to be about the T-34. Funny how even a volatile topic like that can stray into other volatile discussions.