I like the sound of Manchester, but I'm thinking
of the connection
to foreign airports. The amount isn't as high as in London, and
London to Manchester is far.
According to
http://en.wikipedia.org/wiki/Manchester_Airport , the
airport is "the largest UK airport outside London", and is the fourth
biggest. It's only about an hour by plane between London and
Manchester, and would probably be an easy transfer for most people.
If that's not going to be good enough, then realistically we want to
forget about everywhere outside London.
Mike