I think it was more about economics than competency.
By the end of WW2 the UK was bankrupt and completely ruined economically, while the US had become the industrial powerhouse of the world thanks to abundant resources, manpower, and the fact that the war largely took place far from its borders (a few tiny islands notwithstanding). By the end of WW2 the US owned nearly all the gold that Europe previously owned which led to the US Dollar the worldwide reserve currency.
By the end of WW2 the UK was bankrupt and completely ruined economically, while the US had become the industrial powerhouse of the world thanks to abundant resources, manpower, and the fact that the war largely took place far from its borders (a few tiny islands notwithstanding). By the end of WW2 the US owned nearly all the gold that Europe previously owned which led to the US Dollar the worldwide reserve currency.