GLA runway is shorter than BFS, which in turn is around 200m shorter than Prestwick. A 777 to Chicago should have no problems out of BFS.
The 777 entered a brief hold at FL100...but was much closer to BFS at that point than GLA. Also, United operate out of BFS, so would have staff available...although not sure if that was a consideration or not. Do they normally operate from GLA?
Also, seems it was a disruptive passenger rather than a medical emergency.