In terms of real facilities I'm struggling to think of more than two. I guess Manchester must be tops. Perhaps B'ham comes next if you consider their outdoor and indoor "spotting" areas. It all depends on what you want. If you're a number cruncher there may be a few more. If on the other hand you're into photography, then very very few offer anything that's any good. It's usually a case of finding unofficial locations near the approach to the runways.