I will leave school with 16(not counting double awards) GCSE's because I took 4 of them early and got straight A's. My mock results (I posted earlier this year on forums) were mostly A*s/A's too.
As you can see I found them quite easy but I do have friends who got all below Ds in their mocks errm...
As for AS/A2 levels, well I'm doing History A Level at the moment, and I find it's a bit different than GCSE. The latter was all about learning lots of facts and then repeating them on paper... A levels a lot more analysings, possibilities and interpretation of history - a lot more depth and independant study.
I think I will be taking all Sciences (Physics, Human Biology, Chemistry, Math) for my college years (in short what I need to get into university for Medicine).
In any case, don't worry, personal advice is you should make a list of all your subjects...
Find your strongest and weakest, then prioritise your studies. Find subjects you might be on the borderline for passing (like D/E subjects) and try focus on getting them to a pass level, even 11 C's are better than 2 As and 7 Es for example lol.
Good Luck!
Turin