For law, there aren't any subjects you actually NEED. Some universities say that english and history are beneficial, but I haven't seen anywhere that states that you NEED them (Oxford/Cambridge may be exceptions so if you're aiming for those then check them out first).
My subjects are completely irrelevant, yet I received offers from all universities I applied to. (I took physics, chemistry, maths, information technology, english and french - english being the only subject remotely relevant to Law).
However, if you're looking for related subjects, then I recommend: english, history, latin (if you can do this), philosophy and perhaps psychology (just suggestions - not all of them).