Back then, the Nazi's would have had time to get a strong toehold in Britain before we could get there. They didn't have long distances from which to bring supplies, etc. You think supply lines in Iraq were in difficulty? Just imagine what it would have been like for us to maintain a supply line across the Atlantic. We would have have to bring more carriers from the Pacific making that area of operations less efficient and dragging thewar in the Pacific out. We would still have won, but it would have been long, bloody and costly for us and for the Brits.
That...is the key. In the program, they talked about the US staying out of it through the occupation of Britain. I do not believe this would be the case. Obviously, it wouldn't be easy, and it would be costly, but I think we would have made our move the moment Britain was invaded. Whether we would have succeeded in stopping them from actually occupying for a time is another thing, but I guarantee American troops would have been on the ground there fighting before the occupation would have been completed.