You really need to be sure that you'll achieve more than just one A at GCSE if you're thinking of applying to those Unis... Especially for medicine.
Maths is not just looked upon kindly by universities, it is a highly-regarded subject, maybe even the most highly regarded "hard subject." For medicine, it's a given that you'll have Math, if not Further maths, with A/A* at A level, not GCSE.
I don't suggest you do Art. Maybe History or a Foreign Language, that would seem "better."