The massing of the invasion force would have compelled Roosevelt to send U.S Naval forces to the Channel. At this point the U.S. would have been at war with the AXIS partners and the status of our forces at Pearl would have also been different. Hitler made the war longer by not invading.
In any event the Germans were massing barges in the channel ports and the RAF Bomber Command was targeting them right along. How many of these already inadequate barges would have survived to embark troops once the order had been given? Probably the first visible sign that the invasion threat had irrevocably passed was when the barges ceased concentrating.