Price, the difference the last two weeks of English summer hols from Glasgow & Edinburgh can be half of what is charged at Manchester. It's the same for the first two weeks of the Scots hols hence the attraction of Manchester for Scots. Also from having a daughter in English schools all her mid-term hols are different actually the only hols that are the same is Xmas.
I know folk likewise who will head south. I have one friend who travelled all the way to Liverpool from just outside of Glasgow simply to get a cheaper flight to Majorca.