I do not take History, morerather your 3 subjects, with Chemistry and CompSci (Self-studied), on top.
I would say, if you're predicted an A^ in Further Mathematics GCSE, you are more than capable of handling the demands of A Level Single Mathematics, and as someone who, from the looks of it, likes to challenge themselves and take it one step further than the norm (i.e your taking of FM GCSE), I feel that you possess the capabilities and satisfy the prerequisites to take FM A Level, it is by no means an easy A Level, but if you're getting predicted 9s and A^s at GCSE, it sounds like there's nothing to worry about on this front, if you enjoy Mathematics or seek to enter any field of Mathematics, I'd highly suggest it, it's fun.
Physics, on the other hand, is more of a love it or hate it ordeal at AS Level. The first thing you will say is that the AS course is quite dry, that's what I said at first, but as you advance through the modules they not only improve in rigour but they also delve into more interesting content, our class recently finished the 'Quantum Physics' Module, looking at Quantum Theory, and various hypotheses and fun equations. My personal favourite was the Electricity Module, but each to their own. A2 Physics is certainly more rigorous, and has all the big name topics, Astrophysics, Nuclear Physics, you name it.
May I be the first to say that you will rarely feel the weight of 4 subjects including FM being 4, it feels more like 3, and I'll explain why.
1) Your school's FM policy may vary, they may want you do take Single Mathematics AS and A2 in the first year, so you will only really be doing Single for the whole year, and then Further in the next. Please check with your prospective sixth form's mathematics department to find out more about their policies, the policy for my school was to finish AS Mathematics in the first 2 months, and then do AS Further Mathematics for the rest of Y12.
2) The policy may be to teach it in parallel, do not worry, AS Further Mathematics is designed to be no harder than AS Mathematics content wise, their difference is the content delivered is much, much different. When I began AS Further Mathematics, it was when I was still finishing AS Mathematics applied (Stats and Mechanics), and more often than not, it felt merged into one subject.