Tell me exactly which subjects you intent to pick up in A levels and what field you want to do after your A levels.
I took up Physics, Computing, Maths and General paper at A levels.. After A levels when i started applying for Engineering universities, most Universities didnt accept Computing and they instead wanted Physics and Chemistry along with Maths. So if you want to take Engineering after your A levels, then it doesnt matter if you take Computing A levels or not.. But you MUST take Chemsitry and Physics and Maths because otherwise most universities dont allow you to sit in the Engineering test for their universities.
So if you're going for engineering, Physics, Chemistry and Maths are a must. If you think you can handle more, then also take Computing. AS computing is even easier then O level Computing.. But A2 computing is very very hard. I got 95% in AS computing and then after A2 paper my Average score came down to 86% in A levels. So i had probably got 77 or 78 in A2 something like that (A2 results arent shown, instead they show the aggregate of A2 and AS after A2 papers)
Anyway, also if youre going to apply abroad then concentrate very very hard on your AS papers because most abroad universities give you admission based on AS results because you apply way before your A2 papers.