Admittedly I didn't take Biology for my A levels but I did take further maths and honestly, it was definitely the best A level choice I made.
I'll post two messages of different length so you can decide which one you feel like reading.
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The LOOONG message (Read at your own risk...this one might get technical)
Keep in mind that if you pick further maths then you'll only really start studying the modules like FP1 and M2/D1/S2 later on in your AS year (at least that's how my school did it). Meaning that if you understand C1/C2/M1/S1 then those FM modules should come very naturally to you.
The only real shock you can expect by taking FM is at the start (this would happen even if you didn't take FM as mentioned above) when you adjust to A level maths as a whole.
And I suppose the jump from AS FM to A2 FM is noticeable but if you're continuing FM then it's probably because you know you'll enjoy it.
The maths you learn in the regular maths course is interesting especially when it get's to C4/M2/S2 and throughout the core modules you'll advance your skills in calculus, trigonometry, algebra, vectors...
FM's pure modules (FP1, FP2, FP3) will both supplement those topics and then add some completely new ones which you may not have heard of.
The best two topics in A level maths come from FM at A2 imo. (I'm gonna' catch a lot of flak for this...)
-Differential equations
You're already used to solving equations for x (and y if simultaneous) but here the solution will be a function. This is greatly explored in FP2 and used from M3 onwards for describing complex motions though it will make a brief appearance in C4.
-Complex numbers
Essentially this revolves around the idea of describing 'imaginary numbers' involving 'i' where 'i' is √-1. It's introduced in FP1 and at first it seems abstract and pretty pointless stuff however at FP2 you'll see that it has some very real uses to it. Favourite topic of all time imo, hence my name.
The additional applied modules will not use techniques learnt in FP1/FP2/FP3 but will use skills learnt in C3/C4, namely calculus and vectors and trigonometry.
Mechanics, in particular, capitalises on the core skills the most and you end up solving some pretty difficult stuff by M3 but it's very rewarding.
I just think that without FM, regular maths would feel rather anticlimactic since you learn all those neat skills but then don't get the chance to apply them.
Sorry if this has dragged on a bit.