Likely the British Commonwealth forces would have been unable to gain a full victory over the German forces. The British did a magnificent job with what they had until the US forces joined them but just were not strong enough for a killer blow to defeat the Germans. For 2 years British Commonwealth forces on their own prevented Hitler getting full control of North Africa, the Middle East oil wells, defended itself from German plans to invade Britain, had stopped the German atomic bomb research, sunk all Germany's super battleships, broke the supposedly unbreakable enigma code, and other important things which held the German war effort back a lot all while America sat on its hands and refused to get involved. If Britain had surrendered it would have meant no place for the US forces to launch bombing raids or an invasion from making it impossible for the US to fight back.
Few Americans realise Germany had an atomic bomb research program before the Americans had even thought of the possibility of atomic weapons. Likely without the British the war would have gone on several more years. During that time Germany likely would have defeated Russia and had unlimited resources and oil and would have built a super military. The Germans would have perfected their atomic bombs and would have also perfected long range jet bombers to deliver them to the east coast cities of the USA. Imagine it is 1946 and so far America has largely been out of range of German aircraft. German U-boats menace US shipping sending huge tonnages to the bottom of the sea. One day out of nowhere comes a group of the new long range German jet bombers flying at 550 miles an hour. No American plane can catch them and American cities believing they were out of range of German aircraft have no anti aircraft defences. The planes split up and head for a city each choosing cities like New York, Washington, and Boston as well as other major cities dropping an atomic bomb on each. At the same time as Germany now owned Russia they had captured the Aleutian islands and had air bases there and sent in atomic bombers to destroy the cities on the western side of the USA. In one day they could have devastated America to the point the USA would have seriously had to consider surrender.
Americans generally think they won the war for Europe but in fact although they were a great help but they were not the major force fighting in Europe. Only 40% of the soldiers who stormed the beaches of Normandy on D-Day were American. It was Britain and American air forces that bombed Germany's manufacturing to nothing not just the US forces. It was a combined effort of Britain, Russia and America that beat the Germans, but the fact is it needed all 3 to win as Germany had been gearing up for war all through he 1930s and perfecting the ways of fighting a modern war. Britain and Russia did more than America in Europe and the fact it won the Battle of Britain and stopped the German's capturing the Middle East oil wells made it possible for America to walk in to a European country and fight from in relative safety that gave them any anility in Europe. Had Britain or Russia fallen and been captured likely America could have been fighting right through the 1940s and could have lost.