I imagine you'd need to be normally resident at the time you're applying to university, which is a bit more difficult for the average 17-18 year old busy with their final year of school.
The university would presumably also expect a Scotland-resident student to take Scottish exams at a Scottish school, which is a separate system to England, so even more hassle.
The university would presumably also expect a Scotland-resident student to take Scottish exams at a Scottish school, which is a separate system to England, so even more hassle.