No the UK (or rather the British Commonwealth, with our Polish, Czech and Free French etc allies) would not have defeated Germany on its own. But Germany would not have defeated the UK. Even if the Battle of Britain had been lost, there is no certainty that Germany could have gained and retained control of the English Channel with the vastly superior Royal Navy in the way. Germany had very few of the kind of specialised landing craft that the Allies had in 1944, especially ones capable of carrying vehicles.

I think that the situation would have been very similar to 1812-15 - wait while the bad man has run out of steam in Russia and then get a few allies in to help finish him off.