Absolutely no criticism of you, SAM-man, but your school is letting you down badly. It's not the only one.
Single or double award GCSE science isn't really up to scratch for the brighter pupils. You should be able to do three separate sciences at that sort of level. Unfortunately most schools don't have the resources or timetable space to teach them. Or, like most independent schools, they don't want to teach them because if the bright pupils do double award, they all get A*s, whereas if they did three separate ones, they would learn more but might get As or even (heaven forbid) Bs, pushing the school down the league tables.
Not done in the interests of the pupils, but of the senior management team getting their promotions/pay rises.
Tim