Lend-Lease made it easier to afford what was *bought* from the US. But unless the US had flatly refused to trade with the UK, the UK would still have kept the convoys running.
The reverse wasn't true of Germany. After serious early problems, the British stomped the Germany submarine fleet with technological superiority. Combined with the fact that the British also ruled the skies (again through technological superiority) and that the British surface fleet was pretty damn good, the German surface fleet contributed little to the war effort except by sapping some British effort in destroying them. So Germany had no way to get supplies any way except overland or through the Med - in other words, Africa or Asia. Asia was out because the Russians beat them big style. And after a whole lot of grief, the British beat them in Africa too.
Without the Eastern Front, things might have been very different. A possible trading partner to supply coal, oil and ore, and a whole lot of spare troops, could have made a big impact. Still though, winning the Battle of Britain and British superiority in the air (not through better planes but through better planning) meant that any amphibious assault would have taken massive casualties.
A more interesting question is what America would be like if it hadn't got involved in the war. Always an insular nation, a decision to stand back would have left the rest of the world to Stalin, Hitler and the Japanese imperialists, and living in that global environment is going to have an effect. Could Rosa Parks and Martin Luther King have had any effect (or even survived) in that kind of a country?