Its supply and demand. The more carriers that fly out of an airport, the more competition there is to get bums on seats hence keener pricing from LHR, LGW & MAN.
Population of Scotland is (in comparison to London etc) small and widely spread hence not so much demand therefore fewer carriers operating those routes, with the knock on effect that they do not need to compete so much with other operators on price.
You want cheap flights? Live somewhere with a high population density. If someone chooses to live in the arse end of nowhere, then it figures that flights from their local airport will be higher.
JF