I did A levels in Maths and Physics (around 17 years ago when exams actually taxed your brain).
Maths is a definite must for computer related career.
I also did Statistics exams as well which I found very interesting.
I enjoyed Physics at O level but A level really messed up my head with all those complicated equations. Did not really help in my computer career.
I only did CSE chemistry as I sucked at it so bad. Bunsen burners were fun though.
At college I did a City and Guilds in Applications Programming and passed with flying colours. The language unfortunately was COBOL which is the most painful language you could ever program in. Did teach you a lot though.
After leaving college I walked straight into a top paid computing job in London and am still working there!
These days things are a lot different as most youngsters have a good knowledge of computing from an early age and have that advantage over us oldbies that had never had the internet or anything.
Anyway, good luck on your A levels whatever you decide.
Boo!