You piqued my interest with that question so I looked it up:
The British Olympic Committee has rejected calls for the nickname to be changed to Team UK, arguing that neither Team GB nor Team UK are entirely accurate given that neither term covers all the members of its association, and that Team GB is an "effective trading name that fitted best with the Olympic identification of GBR".
https://en.wikipedia.org/wiki/Team_GB
Not sure if it even answers the question though.