Dunash: I doubt David Irving has much to offer on the subject. Not only is his other work being ravaged of late, but the entire German contingent in France when the Western Allies invaded would not have replaced what Hitler lost to Bagration and he could hardly have taken them all out with the US and England still in.
Zoomar, had England done that, the US response would have been awful. Likely the British lose their colonies in India and Asia by 1948, and the Middle East collapses by about 1952. The economy, deprived of Lend Lease and then the Marshall Plan, and possible seizure of Brit assets, would be back in the Great Depression. Meanwhile the USSR would likely have advanced to the English Channel. To give an idea of the USSR's advantage by then, the US and allies faced approximately 50 German divisions on D-Day. By the end of 1944, the USSR was able to swallow 32 German divisions in Bulgaria and Romania as if they were appetizers.
Even the US alone could have maintained the front in Italy and the most that Germany could have withdrawn elsewhere would likely have bought a few more months, even recognizing further German production. Stalin likely pushes to the Elbe by June 1945, and about the time we force Japan's surrender, the Red Army is nearing the English Channel.
Aktarian, you mean after Bagration had begun, also known as the destruction of Army Group Center? Don't know how strong the Germans would be in Normandy if they were trying to recoup from THAT disaster a week before we invaded.
In the aftermath, if we were fortunate, we might salvage Italy, France, Benelux, and the Rhineland for the West.