Semi-relevant Wikipedia:
When Britain reached out to the US asking for help in the war, the US offered help contingent on Britain decolonizing post WW2, and that agreement was codified in the Atlantic charter. The decolonization of Britain (post war) also meant that US and other countries would possibly have access to markets to sell goods that were previously under British Empire-which was not accessible to them then[86][87] To bring about these changes, the establishment of UN following World War 2 codified sovereignty for nations, and encouraged free trade. The war also forced the British to come to an agreement with Indian leaders to grant them freedom if they helped with war efforts since India had one of largest armies.[88] Also, following WW2, it was untenable for British to raise capital on its own to keep its colonies. They needed to rely on America and did via the Marshall Plan to rebuild their country.