Whichever you feel most comfortable taking and thing you'll do best in, really.
Around four or five medical schools(including Newcastle/Durham and UEA) will only ask for chemistry or biology, most will ask for both. So if you want to chose from as wide a range as possible then it's probably best doing both biology and chemistry.
The rest of your choices can be from the arts, social sciences or sciences. So History, Geography, Philosophy, English Lit, Modern Language, Physics, Maths....
So along with Bioiolgy and/or chem, just chose the one you think you'll perform strongly in/have the most interest in. Try and keep it as traditional as possible (A-levels like Critical Thinking and General Studies are very unlikely to be accepted).
*I'll try and come back to this later to offer a fuller account.