im only a gcse student doing my gcses right now, not sure if any of this is helpful but im in the same exact situation as you were in year 11 right now, following the exact same timeline from taking a great interest in medicine last christmas and having the exact same a level subject choices to now reconsidering to swap, except for me it's biology to computer science. feels a bit too coincidental i must say
ive some some research on this. you can get into medicine with just chemistry only, and i plan on keeping that because i quite like it and as a side effect also keeps my options open in case i decide that i do want to actually do medicine. though it's only like 2 or 3 med schools that make chemistry required and biology not required. buckingham is one of them i think.
judging by your love for maths, im think probably land an easy A or A* in further maths given you dont seem to find it mundane at all, ive experienced the link between enthusiasm and grades first hand with physics first hand. im not sure how swapping to further maths midway would look from the universities' standpoint pessimistically, but optimistically i think it would look quite good since you'd have realised what you really want, and also you would have obliterated the course which also happens to be one of the hardest at a level. it proves your enthusiasm and your skill. do you have a careers advisor at your school? they might be able to help you understand what unis might think.
you might also want to learn some further maths in your spare time to see if you can do it and land top grades, though im sure if you can if you're doing A*s in maths now (don't take my word though, i only think so based off the limited knowledge i have about the course difficulties).
you should also ask your school if you actually can swap though. I'd guess they'd be reluctant to an extent, but if they are you should really plead. maybe cut a deal if you have to
also, if you're interested you can still do computer science at university with your maths a levels. maths is the only required subject in most unis as far as ive seen. im only swapping for computer science because i have access to it and im really interested in the nea work (and also the course content of course but the nea is the USP for me), though there's no strict need for me to do so in order to do CS at uni. i reckon you'd be very into data science and ML which is just fancy statistics