I guess it may also be that the Scottish unis are highly prized so, for many, no need to travel. You might be regarded as a social pariah if you came from Glasgow and chose Edinburgh (only mildly joking there), for example.
The exception is St Andrew's.Mainly English students there. When I was at school not one of my friends applied to St Andrew's.
I am sure there are a range of reasons why people desire a new location. the desire to stay within easy travelling distance is certainly disadvantaging certain - very fine- unis. Even some young people choosing between Cambrisge and Oxford choose Oxford because it's easier to get to (and out of!)
I have definitely noticed at offer days and Open Days that most accents are at least reasonably local to the uni. The exception actually was Aber. I guess even for Welsh students it's not very accessible!