In the US they just set the QNH of the airfield within 100miles of where they are flying - seems to work OK for them.
As for setting airfield rather than regional it might be because of the small size of the island. Suppose Coningsby had one QNH and Waddington another? Which is better?
So KCLT and KPTI....50 or so miles apart...both International Airports....might have a different QNH....to what effect? How much variation do you think there might be....and what effect upon altitude conflicts would it have that would matter?
Add in Altimeter Error which my foggy memory tells me is limited to +/- 70 feet....and you still should not have a problem.