Britain saved itself during the Battle of Britain. But it took the US to defeat Nazi Germany.
Hitler was ok with Britain being neutralized, while he turned his attention East.
It took the COMBINED efforts of the three great allied powers to defeat the Nazis. NONE of the three could have or did do it alone.
The British needed Canadian and American, as well as Empire, help and resources.
America, in NW Europe, Italy/Sicily and N Africa, in fact relied greatly on British manpower and resources, much more than people either side of the ‘big pond’ realise. Even despite your truly massive ability to create and move men and material.
And the Soviets wouldn’t have defeated the Nazis in the East without the utterly crucial Lend Leases of America and Britain. And we Limeys and Yanks and Canucks had the help of the Soviets taking on the vast majority of the German army eastward.