I have a friend who's planning on doing the same, and dropping both maths at AS level as we can still get away with that, I personally prefer to be rid of the Biology and just take 4 of the best subjects it is possible to take. I also know, however, that universities give out just as many places to people studying 3 subjects as 4 and if you take 4 or even 5 they will give you an offer based on those rather than just three of them. So if you did maths, F.maths, physics they might offer you a place for A*A*A (that's what my brother holds from Cambridge at the moment with chemistry as the A and the two maths together), whereas you take 4 subjects if you consider sometimes they flatten maths and F.Maths to just F.maths, but for the sake of the example they haven't, You might need to get A*A*AA, which is still ridiculously hard and probably not worth the extra effort when you can get in with fewer A-levels.
I have also been advised unless you take maths at uni F.maths is ignored or accepted as simply maths, so if you don't want to go into maths then perhaps only take the AS of F.maths.
If you plan to take maths I would recommend dropping biology, unless you really enjoy it.
If you're taking chemistry depending on which type you want to go into, you might want to drop either physics or biology.
If you want to take biology I would recommend dropping physics, equally the other way around if you're taking physics you really don't need biology.
Although it's up to you I would drop at least one of those before September, and depending on what you want to do possibly only take AS F.maths.
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