Hitler's entire motivation was to secure lebensraum in the East. Other than humbling his country's rival, France, he had no real interest in the West.
Also, building a navy is much, much harder than building an army. Capital ships are incredibly expensive. Only nations safe from foreign invasion can invest the expense to do so (generally island countries like Great Britain or Japan and the United States which achieved decisive control of North America by 1850). And building a naval tradition is hard work that requires prolonged experience and know how. Germany had just started building that around 1890 and then had it interrupted for about 20 years. It's not something that can be instantly created in 5 years or so.
If Germany did concentrate only on Britain, the Mediterranean becomes the decisive theatre. There is a strong possibility that Japan still moves south in 1941, and the Germans and Japanese will attempt to link up in Asia. It will be very tough for the UK (and eventually the US) to hold out, but they can do so. The Allies probably concentrate on taking out Japan, which they will do in 1945/1946. In the European theatre, they probably only have North Africa and some Mediterranean islands when the first atomic bombs are produced. Then the Allies rain atomic holocaust over German cities in 1946/1947 and land troops ont he continent only when the German economy has completely collapse and the Reich is in chaos.
Of course, that is assuming the Soviets do nothing. Stalin is not much of a gambler. He wants easy wins in collusion with other great powers. If Hitler does not move against him, I don't see why he wouldn't agree to Stalin's idea of the USSR joining the Tripratite Pact - which may mean the USSR enters the war against Britain and takes Persia. In which case, all bets of an Allied victory are now off. Or, Stalin sits tight until he sees Germany begin losing in 1944/1945. Plans for an attack are made, and after atomic bombs fall on German cities, he strikes pushing as far west as he can.