It solely depends on interest in my opinion.
I'm taking the same subjects you're taking. I'm doing A level maths this year, and FM next year. AND I'm self-teaching.
But because I'm so interested in math, I average 97-98% on every past paper I solve. If you're not heavily interested, don't take it.
Also, if you don't think you can get an A/A* in A level math, don't bother doing FM