If you want to study a Maths degree you should want to study Further Maths at A-level for a few reasons. But if your school doesn't offer it, fair enough - perhaps you could study AS Further Maths in your own time, as well as mention on your personal statement (or have it mentioned in your reference, I'm not sure) that you'll be studying FP2 and/or FP3 in the summer before uni, and do it - it'll come in useful at uni.