The British Empires global monetary dominance only ended finally as a result of the Suez Crisis.
When USA, now dominant militarily and in trade refused to back the British against the Egyptian nationalisation of the strategic canal.
An empire no longer able to control significant trade routes was clearly no longer a dominant entity and so the 'good as gold' British Pound was dumped by central banks and traders globally, being replaced by the USD.
It was around the same time the CIA got involved in removing the democratically elected government of Iran to install the useful puppet shah who would not nationalise UK and US oil interests in Iran.
During the transition from British imperialism to US imperialism there was considerable co-operation between the ascending and descending empires.
It is interesting that today as the US empire is declining that its 'Suez Crisis' equivalent may well be the 'Hormuz Crisis'.
Empires get tired, over extended, outdated and superceded.
Their arrogance built upon 'extraordinary privilege' tends to prevent them seeing what is happening...until its too late.