Hitler didn't think he needed to defeat Britain, he just needed to defeat Churchill. Hitler thought the British were his natural allies and if he could get rid of Churchill, someone like Chamberlin could take power and would quickly sue for peace. Then he could focus entirely on the Soviet Union - maybe even get the Brits to help him.
If the RAF had lost the Battle of Britain, it's questionable if Churchill could have retained power and Hitler might well have been correct...