The allies had surprisingly few plans on any invasion of Nazi-Germany. There was a vague plan to invade in 1941 had it not been for the German attack in 1940. However what we do know is that it probably wouldn't have come this far. It should be remembered that Hitler's position before the battle of France was not as firm as it would be after the battle. There were many high ranking officers in the military who were planning to depose him if the invasion of France failed. Therefore (and this is speculation) it's more probable that had the invasion failed the German Heer would have taken over, deposed Hitler and formed a peace with the British and French. My primary source for this is ''The fall of the third republic'' by William L. Shirer.