Parents like this baffle me. Barring you from subjects you're good at is like sabotaging your application. I'm sorry you're going through this. Maths is an amazing subject for a med school applicant, and it's one of your strengths. All candidates applying for med school will lean into their strengths heavily and if Maths is yours, then do the A Level. I don't understand your parents' logic. I saw so many successful applications with the Maths A Level. Actually, that seemed to be one of the common subjects! Biology, Chemistry and Maths is a great subject combination.
With how stubborn your parents are, I have to ask. I'm not trying to be rude but is Medicine even a course you want to do? Or is it a course they chose for you? They're acting like they're the ones going to read the A Levels and go through med school. They need to give you space to make these decisions.