It was very difficult for Hitler to conquer the UK, given the small size of the Kriegsmarine and the large size (and experience) of the Royal Navy, not to mention the excellent RAF. I doubt he'd ever have managed it, even without opening an Eastern Front, but I do think he might have made things difficult enough for the British to eventually arrive at a fairly satisfactory "mutual arrangement" with them after a few years of stalemate which could have temporarily ended their war without either side losing it.

How could he have done that? Well, he needed to put all his effort into driving the British out of the Mediterranean Theatre. This he might have managed if he'd left Russia alone and made a much more serious effort to take North Africa, Malta, Egypt and the Suez Canal...and maybe Gibraltar (but he'd have had to convince Franco to help him for that...or invade Spain). With Egypt taken, he could then have moved forces into the Middle East to get the oil there, and the British would have been forced to reroute all their Asian trade traffic around the Cape of Good Hope.

Given such a situation, the British, clever pragmatists that they always are, might have felt it better to bargain with the Germans and arrive at an arrangement where Hitler holds western Europe and they hold the British Empire, get back some of what they lost in the Med perhaps, and the fighting stops.

Just possibly this could have happened with a successful enough German-Italian campaign in the Med.

But it would probably have led to further fighting with Britain and the USA at some point not too far down the road, and the Americans and British would have joined forces, figuring Germany was getting too powerful to tolerate.

Hitler's original plans for war in the West were predicated on not starting that war until 1944. He felt his navy would be ready by then. So the start of the war in '39 was not what he expected at all, and it derailed the plans he had to build a sizable modern navy that could challenge the British and Americans.

He really didn't think they'd fight him over Poland. That was another one of his big miscalculations. (From his point of view, he couldn't understand why they would even bother fighting on behalf of the Poles! He did not appreciate the fact that Britain and France, countries just as proud as Germany, had simply had enough of his bullying tactics by that time. Of course, he thought he was "the good guy". The usual thing people think. He regarded Poland as an abberation. It hadn't even existed as a political entity when he was a young man...half of if was in Germany then and the other half was in Russia. This, to Hitler, was the normal state of affairs, and the new nation of Poland that arose after the Treaty of Versailles was an "abberation", a land theft from Germany and Russia, a government which had no right to exist. Stalin felt the same way. That's why it was natural that the Germans and Russians decided to divide Poland between themselves in the Fall of '39.)

The Poles, of course, didn't see it that way!