The British (and French) owed the US for WW1 and the Nazis were financial trolls who didn't pay back Anglo or French debts because they wanted to fuck over the lenders (who they felt were Jewish).
The US had lent out over a $1T to the Allied powers for WW1 and that's why FDR supported England while the US was neutral - losing that much money would ruin any country in 1940.
The cause of the rise of fascism in Germany after WW1 was determined to be the austerity enforced on the Germans in order to pay back the reparations. Investing in Germany was mandatory to eliminate the rise of another populist dictator.
This was the second war Britain got into that it could defend itself abroad without the US (Singapore was an embarrassment for Britain and the Burma campaign is underreported but showed the English were still fighting in their colonial style of using poorly trained irregulars that repeatedly cost them supply and initiative). It became clear to the US that the British could not afford another war and we're in no position to rebel against any judgement imposed on them.
Britain was also losing her own colonial nations that had nothing to do with America. India was never going to remain a British occupied region. Huge swaths of Africa were decidedly outside her grasp. Most of the middle east was also anxious for their own independence so Britain never stood a chance of being their Victorian size for long.