In what respect, Slag?
I don't say that the UK could have defeated Germany. Most likely, at best they could have clawed back some of France. But the German invasion of Britain was never on the cards after the Battle of Britain - air power trumped sea power. Crossing the Channel gave a massive home advantage to the defenders on either side, but British Fighter Command was massively coordinated for intercepting enemy aircraft. The British were also the world leaders in radar - no other country had implemented a similar radar net, nor had such advanced detectors. It's also worth remembering that although the Spitfire and Hurricane were instrumental to the RAF's success in the Battle of Britain, later aircraft like the Mosquito were far more potent and outclassed their German equivalents.
The battle for Africa was a battle for control of oilfields - both sides realised that without oil, mechanised warfare simply couldn't carry on. The British and Commonwealth troops won it without American assistance.
Code-breaking allowed the Allies to direct attacks at the most significant parts of the German war machine. This was achieved using Polish intelligence and British engineering, without American assistance.
Convoys from the US provided vital support - raw materials like oil and coal, and industrial equipment and weapons - and without the convoys Britain might well have run out of resources to continue fighting. They weren't free though - Lend-Lease required payment, even though that payment was deferred after the US entered the war in its own right. And Britain would have continued convoys regardless, because they *needed* that stuff. A successful German blockade *would* have worked, but the British navy's anti-submarine warfare outclassed the German submarine fleet through improved detection, developed by British engineers.
What Britain lacked, in short, was manpower, money and raw materials. It could defend itself, but it didn't have the resources to *project* a lot of force. The US provided that. Britain probably couldn't have won on its own, but equally it probably couldn't have been defeated either.