The Scottish system is, as you say, different.
They do their important exams in S5 (Year 12) - and lots leave school at the end of S5 to start university at the age of 17.
I expect the best option for you would therefore be to go into S5, take 4 highers, and then leave for university. You wouldn’t need to do S6 too, so you’ll still finish school at 18.
However, this seems like a very strange choice to make, and not the best option for your education. You’ll have to switch to a completely different educational system and different exam boards/curriculums for your subjects. Finishing your A-levels in England would be a lot more logical - can your family not wait one more year before moving to Scotland?