I would do psychology! It's a fascinating subject and considering you're doing biology and chemistry, it'll give you a strong advantage when studying neurotransmitters, disorders and drugs. It's very easy to tie it with your other subjects, which is good for when you write your personal statement. My friend took biology, chemistry, maths and psychology and got all her offers so if any one tries to make out that it's not a respectable subject, complete and utter lie. :P You could even make a tie with psychiatry, which you'll study on a medical course. So if you like psychology, it might influence what area of medicine you want to go into!