America built the trucks and supplied the grain - and made a profit whilst doing so. We only just finished paying off our war debts to the US
Britain also fought the Japanese at the same time as the Germans. Except on AAR land the worst that Britain would do without American help would be in armistice with both combatants which allowed Germany to fight against Russia alone and Japan against China. Wars that even without US and UK aid (something that would have been crazy for the US and UK not to give, even in this alternate timeline) both Fascist powers were not assured of victory.
Japan would not have taken India (and couldn't have held it anyway) and Germany could not take the UK.
The Manhattan project would not have been completed nearly so quickly without the assistance of non-American scientists. And Britain was reasonably adept at building heavy bombers, certainly capable of modification to carry a nuclear weapon.
As for the Soviet Union, again the affordability of the cold war was linked to the American military-industrial complex powering economic growth and the trade blocs built by America to keep the communists out.
Britain (and other nations) had faced tyrants before and beaten them. It may have taken longer and been more painful but tyranny does not endure. As is shown by the history of the USA.
On a final note, we are all grateful for the part that America played in defeating the tyrants but it was only a part. To suggest that America and only America was principally responsible for winning the war and preserving the peace afterwards is why America and Americans can be so disliked in some quarters of Europe.
Real life is not Hollywood. In real life the 20 million Russians who died facing Hitler have as large a claim to ensuring our freedom as the USA. As do the millions of Chinese who prevented Japan from rolling over the richest country in Asia. And yes the American industrialists who armed and financed the eventual defeat of the fascists have a claim but only a share.
God bless America and god bless all the peoples of the world fighting tyranny.