Hey, of course am not a teacher (so disclaimer).
MY main point: YOU REALLYYYY DONT NEED TO DO MATHS. Trust me, and more importantly trust the university course overview pages!! They all say very clearly that they *require* biology and chemistry, and anything for a third subject (maybe some want maths, idk, but check what their exact criteria). You're only job is to follow their exact criteria when doing your A levels. Doing Maths and getting a B (as this year showed; people who were getting As all year suddenly, unexpectedly got Bs/C's because of how ridiculously high the grade boundaries were) means no medical schools (near none) will accept you and reduces your chances of getting in. On the other hand, doing a subject you enjoy and that comes to you more easily, and getting an A* (whilst getting an A in bio and chem) makes you SO much more desirable to admissions team. Like, SO much more.
I think you should trust what I'm saying because I kind of had the strong belief that four A levels would make me a more competitive applicant (to whatever), so I stuck with it. I got good grades, but I KNOW if I'd dropped one and focused on 3 it would've been better; if I'd just actually listened to what the actual universities advise, it would've been simpler and less stressful for me. I only really saw that I could've taken 3 and had better level of success once I'd finished my A levels/was sitting my exams.
My point is: the universities on the most part explicitly say what they need from you; if you still don't trust it, call up admissions and interrogate them until you're satisfied. Don't take maths, a subject that will require 1000x your effort for a not-so-brilliant grade, whilst you can take something you'd enjoy more and get a brilliant grade in (hopefully!)/more likely to get a brilliant grade in, and STILL be a good candidate for med. I for example got A*AAB, A* in English, AA in biology and maths, but that still made me eligible to apply for dentistry at kings because I had that A* in English.
Another note: I'm "decent" at maths; not gifted but not awful, and it still took me quite a lot of time out of all my subjects. Doing homework, marking it, understanding it, practising takes up a lot of time, especially for someone who's doing 2 other science subjects and extracurricular stuff for med.
I advise: no maths!