The US was helping the UK long before Pearl Harbor via Lend-Lease, just as US small arms companies were making rifles for England before the US got into WW1.

And Germany got very close to strangling Britain with its U-boat blockade.

The question as it stands deals only with the UK, not with the assistance of the Commonwealth forces or any assistance from other nations.

Could the UK, alone, have defeated Germany? I doubt it...Germany would have starved the UK into submission, as the UK alone wouldn't have had the material resources.