Would we have? Doubtful.
The Lease-Lend program was not popular with the general public and keeping it going while we were in a major war of our own it would have been political suicide for Roosevelt. When you read the editorials of the time it becomes clear that after what happened in the Great War we really had no interest in getting involved in another European war. The USS Ruben James incident shows how determined we were not to get involved.
But once Hitler declared war on us the die was cast.
-—Could we have kept helping the UK? Yes.Would we have? Doubtful.-—
I’m not as convinced as you are, but even if you were right, I don’t see any scenario where the Nazis successfully occupy the UK. Before Pearl Harbor, Hitler was already at war with the Soviets, too. He was in deep trouble the minute he did that, even if he didn’t realize it immediately.
The UK couldn’t have invaded France by themselves at that point, but they definitely were going to remain unconquered themselves.