SJB:
I seem to remember that the UK didn't finish paying back it's WW2 debt to the US until something like the early 2000's. So much for 'special relationship'.
During WWII the UK had to pay for arms with gold, so the UKs gold ended up in the USA.
This created an issue where until then world trade was based on the GBP/Gold Standard.
In the 1950's to try and get world trade running again, the $US was chosen as it was the currency with the most gold backing it.
Again, nothing to do with the USA being great...just lucky