Depends what law degree you want to do but... psychology is good for Criminology which is a specific type of law you can do which my friend did at Leeds (joint honors of Law with criminology I think it was), I would also say economics is good as well, history is useful as well especially if you do British history (especially if it is politics), maybe geography or RPE, my Sixth form offers what is called government and politics... does your school teach something similar like politics or government studies.