Do whatever you feel most confident in. Maths A level isn't a requirement for any medical schools if you already have Biology and Chemistry, but if applying to Oxbridge it will increase your chances. It can be beneficial for studying your other subjects (there are lots of calculations in Bio and Chem) and can be beneficial for BMAT, if you decide to sit it. If you took History/Psychology, it wouldn't disadvantage your application, but from personal experience essay subjects can be very time consuming!