There are two major reasons why Britain was able to avoid being invaded by Germany early in WWII. First, Hitler was not really eager to invade England because he thought of them as his natural allies. Second, it was impossible to invade so long as Britain's navy maintained superiority on the sea.
Hitler felt that the British were generally Aryans. Therefore, he believed that they would come around to being allied with his Aryan movement. This made him reluctant to alienate them by invading their country.
Second, Britain's navy was much stronger than Germany's. Therefore, Hitler tried to destroy the British air force first (in what became known as the Battle of Britain) so that his air force could then destroy the British navy. The British victory in the Battle of Britain made this impossible.