@Stickerrocks I don't exactly live in a selective state-education area (aka grammar area). There is a grammar school about an hour away (when counting changing trains, or waiting for buses, walking to and from the station), but 99% of state schools are non-selective comps. There's also a lot of independent schools, which vary in how selective they are.
Some of the state schools have sixth forms, and others don't. Typically the schools with the better GCSE results have the sixth forms. I don't know how that came about - it was just what was there when I moved here. Additionally, there are some 1500-2000 sixth form college options, as well as the sort of places where you go to do Health and Beauty (also called colleges).
If you want to do maths or whatever, then you just decide where you want to do it, and as long as there is a space and you pass the interview (I've never heard of anyone failing the interview) and get the grades, then you are in. There are a lot of options for places to study A Level maths. The SS sixth form college is just one of the options, but its a bit different from the others in terms of how demanding the course is.
DS got 95% in his maths mock without doing any preparation at all. That's fairly typical for him, and I'm not stealth boasting on his behalf, just trying to explain how its not really about sweating to get an 8 (or a 9). I know its unusual. However, I suspect its run of the mill at the SS college. In fact, I believe that DS's maths ability will just be ordinary come September. Maybe he will be in the middle or bottom set.
Everyone else in his current maths class - all of whom are predicted 8s or 9s (I suspect, knowing the school) - will stay on and do A level maths if they want to in the school's sixth form. Those at the other schools, where the school doesn't have a sixth form will go to another school's sixth form (if they can get one of the few places, or go to one of the sixth form colleges and do whatever A levels they want to and are up to doing).