At GCSE, I did combined science as well and got an 8/8 in it so chose biology, chemistry and maths. I also originally wanted to do medicine, although idk what you can do with just bio as I wanted to be a doctor so needed those 3 plus really good GCSEs. In my opinion, biology is the best science and I have absolutely loved the past 2 years doing it. On the other hand, maths has been harder for me but that has been a combination of different factors (e.g. bad teachers, noisy class, not enough structure, etc). From my experience maths has a huge work load in comparison to bio however it is a really useful A level especially for any form of medicine. Some teachers have even said the better the grades at GCSE, the better chance at A level so keep that in mind as well (I've known people who got 6s at GCSE and have really struggled to get anything past a D, but again personal experience). Hope this helps a little and sorry I wasn't more use about the medicine bit!