This question has been studied for a long time and we are not 100% sure. So I will just give you the answer that I consider most correct.

First of all, you have to understand that once you are at a war with Britain you are not gone get any more supplies by sea. This fact is very important. When you look at a map after the 1940s invasion of France, you will see that Germany has not many trading partners left. Germany is locked in between Britain in the west/south and the Soviets in the east. They had to get the majority of their supplies threw the Soviets. In turn the Soviets demand a great deal from Germany. The wanted to get their hands on advanced technology, such as the newest German airplanes and their innovations in chemical technology.

The Germans were of course not very happy with this situation. The realized that defeating Britain would not be possible in the short term. So if they had done as you suggest and went into a long term fight with Britain, the Soviets would have been their major trading partner. Germany would have been completely depended on Soviet supplies. The soviets would during that time, build an up their armed forces even more then they already were at that point. They would also use their access to German technology to quickly improve upon their own technology.

So look at like this, every year Germany spends in a struggle with Britain, Germany would get weaker and the USSR would become stronger. At any point in time the Soviets could stop the shipment of goods and Germany would be in huge trouble. Germany would be completely dependent on the Soviets and would essentially be nothing more then a minor partner in an alliance with the Soviets.

In 1941 they had the best army on the planet, they had enough resources for a major land offensive (largely of course thanks to the USSR). The soviets were still in the early stages of their buildup. Britain was not capable of counter-attacking. When would the change ever be better to defeat the soviets? Additionally with the Soviets gone, their was the idea around that Britain could finally be forced to the negotiation table.

Source:

• The Wages of Destruction: The Making and the Breaking of the Nazi Economy