There's no point imagining until you know whether its feasible to do it in your school. Check that out first. Basically it would mean doing 5 subjects in Year 13. I did 4 and I found that quite tough. I think you would probably have to spend one day, maybe a Saturday, devoted to the A2 Further Maths. I think since you'll have to do uni applications next year, you might be better off just doing the AS level. It may even be worth self studying over the summer one of your subjects to make this practically possible, and/or to assess your capacity to teach yourself further maths and/or other subjects.