Well, the Battle of Britain ensured that the Luftwaffe could not provide air superiority for an invasion fleet that would surely have been pounded to pieces by the Royal Navy and Bomber command. Add to which, British fighter and bomber production actually outstripped German during the same time.
Hitler's decision to turn around and start on Russia gave Britain some breathing space and further German defeat in North Africa made finding oil for fuel a major priority for the Wehrmacht, which meant a further push east, the horror of Stalingrad, the onset of winter etc.
Britain might have held out independantly, but once the Red Army got itself together who's to say the whole European continent might not have found themselves swapping Hitler with Stalin (possibly an even worse monster) with Britain to fall under a Soviet Naval blockade and America thanking their lucky stars that they stayed out of it.
However, it was American troops and equipment that made sure Europe returned to democratic freedom, and this was largely due to a president who understood what had to be done despite rabid isolationists within his own country (hello Joe Kennedy) and the Marshall Plan.
Roosevelt was, IMO, the best American president the rest of the world ever had.