Why was Britain able to avoid being invaded by the Nazis?

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.