Would you have a teacher teaching it, or would you be self taught
If it was with a teacher at school then I'd reccomend Maths, (me and a friend were able to do an extra As at lunchtimes in further maths as we already did maths, but we had a teacher)
However it's probably quite hard to self teach if you're not doing any other sort of maths
You're already doing 5, so if you'd have to self teach why don't you just sit the general studies exams (As & A level means only 8 hours of your time in total)