No, it's mostly decided by your AS predicted grades, and how teachers think you can do, AS actual results only modify this if you do significantly differently from how your teachers predict. Also if your teachers predict you A/B, then the school will predict you an A, or out of B/C, a B etc. Well this is certainly how it works at my school.
Just looking at AS results wouldn't take into account the capacity for resits and the fact the exam may just have been on an off-day or had a Q you weren't prepared for.