Original post by Faction ParadoxI obviously don't know the situation with what A Levels your college lets you take, but I would definitely recommend that if you take up a new one that the one that will be most beneficial is Further Maths. Further Maths AS, which you might be able to take up next year (depending on college policy) wouldn't be able to count towards entry requirements, but would give you a leg up when it comes to MAT (Oxford), CSAT (Cambridge) or STEP 1 (mainly ICL, but all unis top unis will like it if you get it), and it may (althugh I am seriously guessing here, feel free to tell me I'm wrong if you know better) give you a not insiginificant advantage over anyone taking just Maths.
Although if Further Maths is a sensible option, then you can still take things like STEP 1 or AEA and take part in Maths Challenges, as this may be able to help you prove your maths is strong enough to keep up on the top CS courses.
However if applying to Oxford you will have a chance to prove your maths skills in MAT, or if you invited to interview at Cambridge, the CSAT. So taking AEA or STEP 1 (unless it's in an offer requirement) is probably only going to be useful if you take it this year, which would mean putting in a not insignificant amount of effort and time.
(also again, tell me if I'm wrong but I seem to remember that ICL states that if you don't have Further Maths they'll take STEP or AEA instead, which may be something you want to look into)