I don't think that the British resolve ever would have been tipped by a simple symbolic event. Fact is that Hitler was loath to invade Britain no matter what. The only thing that would have put Churchill on a Battleship headed for Nova Scotia was the wholesale loss of the British Isles and simply put to change the preparations of Sealion to the point that they were a credible threat to Britain would involve changing a great deal of what went on in the mind of Hitler.
This then becomes a spculation based on the notion of "what if Hitler wasn't such a strategic imbecile?" or better yet "what if Hitler was a slightly different kind of moron?".
The whole point of the U-boats was to force Britain to capitulate politically to Germany, and that was never going to happen. I think that if Britain was on the verge of Starvation FDR would've found some excuse to save them. Run American ASW and merchants by the dozen through the whole of the Western Approaches, make them impossible to not sink and as a result either the U-boats let a lot of tonnage through to Britain because of overzealous caution or they sink a lot of Americans and FDR gets the political currency to get into the war earlier.
Imagine entire convoys of nothing but American ships and escorts. That'd be a helluva gambit.
