Well, the question is very complex. One discussion could go in debating over invasion of England during 1941/1942. The other could go in debating whether Wehrmacht could survive onslught in 1944. These debates presume no A-bomb occured.

Regarding the first one, I think that with direct or indirect control of most sea countries in Europe, Kriegsmarine would eventually built up capacities to invade England and counter any action by Royal Navy. Let us not also forget that RAF was on brink of collapse in September 1940 (or so) and that Luftwaffe was damaged but not badly.

Presuming that Luftwaffe is on parity with RAF and that Kriegsmarine managed to unload best German armies (6th Army) and Divisions (Waffen SS, Grosdeutschland and Lehr ) there would be quite some panic going on on this tiny island (compared to vast expansions of Russia on -40 degrees celsius...). Further more, BEF took huge beating in France during 1940 and Royal Army was really in no shape to match Wehrmacht's tactical excellence. If Germany managed to unload 1 million men London would be captured and game would be over- that's for sure, at least for that part of story.

Now, onto the second debate, due to never ending conflicts and frostbites Wehrmacht lost better part of it's 150+ divisions that initially invaded Russia and that were of far better quality than those in 1944. Not only of better quality but numerically stronger than those in 1944 (9 batallions in 1941 opposed to 7 bataliions in 1944, 240 tanks in 1940 opposed to 100 tanks in 1944 etc...). And if we look at one principal force that stopped the Allies at Caen for almost 7 weeks that was 12th SS Hitler Jugend and Panzer Lehr. 2 bloody DIVISIONS !!! Others did not prove their worth at Normandy beachhead although there were 50+ divions in France altogether (oldtimers forcefully drafted, divions formed from sailors or lulftwaffe personel etc...).

Of course, Liebstandarte and Das Reich were also deployed but were at that point in time mere shadow of their former self. Mentioned 2 divisions principally stopped English and Canadian Armies for 7 weeks around Caen despite continuous air raids and it's consequences. Others were lost at Fallaise due to poor comand of uncle Adolf. On western front there were some 1 million German soldiers - of quite poor quality if I may say. That were ground forces. As regards to air forces Allies attacked with 10.000-12.000 aircraft. Germany deployeed 1000 (some in need of repair and desperately lacking fuel which was principal problem Wehrmacht and Luftwaffe faced at the time).

In 1944 Germany managed to built 20.000 aircraft staggering feat by the likes of Albert Speer but most of them were wasted on Eastern front. Think how many would have been built in period 1940-1944 ? The invasion would not be executed during 1944 in the first place because Allies, at worst, would certainly have much harfer time in Africa and Italy but probably MUCH later or even more probably never because they, in my opinion, would be beaten in Africa - but that's another discussion.

So, the situtation is now completely different in 1944, Allies still have green divisions as they come while Germans are veterans in experience and leadership and unscaved in Russia that is and there are 3.5 million of them as opposed to 1 million as history knew them. Another plus would be that parity or supremacy in air is achieved. Luftwaffe could easilly deploye 20.000-30.000 thousand fully fueled planes in air, and for example 6th Army with it's 20 best divisions the world has seen (only 6th Army without the likes of Liebstandarte, GrosDeutschland, Totenkopf or Das Reich which each could easilly beat 2-3 Allied divisions) and it's game over for Allies. With Rommel, Guderian, Von Kleist, Von Rumsted, Manstein and even Richenauer (T34s gave him heartattack in 1942) at helm, parity in air and on a ground, the invaders would be swimming without a shadow of a doubt.