I think it is worth appreciating that Operation Overlord was actually plan B. When the invasion, or the liberation of France as Churchill preferred to call it, was first mooted, the plan was drawn up by General Morgan. It is clear that an 'invasion' would have to take place eventually from our Country to somewhere on the European mainland. When Eisenhower, Montgomery and the supreme HQ staff reviewed the plan it was deemed to be too small in content and there was a possibility of it being unsuccessfull. They therefore bolstered almost every aspect of the basically sound plan by increasing numbers, vehicles, ships, aircraft and supplies. The Mulberry harbour was instigated and despite a short delay on June 5th success eventually was reached.
The question should therefore be what if plan B had failed. We threw just about everything into Overlord, which if unsuccessfull would then have become plan C probably some years later.
The atomic option cannot be ruled out. Many historians feel that if all else had faile Germany could have been adequately disabled by one or two well placed bombs. When faced with defeat, as we may have been, it would have been an option - the final one admittedly but when your back is against the wall we needed to survive.