I reckon, as long as you meet the requirements for the medical course (make sure they don't need maths), I think you'll be fine with art. You clearly love it. Besides, a medical student or doctor needs to have hobbies and interests outside the subject. And medicine is one of those careers which (I think) leaves a bit of customisation, by which I mean that you don't need to give art up during your studies and work. The A level changes mean that it's a 2-year course and so, perhaps, harder, but honestly? I think that 4 subjects over 2 years is definitely doable.
Definitely double-check you don't need maths. That's my only concern.
Otherwise, if you think you can handle the workload, go for it! And besides, chemistry and biology are hard so having art there to break it up a bit would help you mentally cope, seeing as you love it.