So BA fly to 20 U.S. Cities, one destination/15.8 million
In the UK, that number's 8, but that's twice the density at one per 7.8 million
Not too bad though, in India it's 5, one per 240 million - in China, 3, one every 450 million, and that's if you include Hong Kong.
But in the spirit of fairness, I think Gibraltar wins on an over-subscription to BA, with one destination per 30,000 people.