Do the subjects you are best at. For law, no specific subjects are required or preferred, meaning -for example- an A in theatre studies will probably be of greater help to you than a B in further maths.
Also I saw somebody said an essay subject is 'essential', this is not true either. That said, doing at least one essay subject will give you a big advantage for the LNAT, if you end up taking this exam.
In short, you do not have to do either. I took History thinking it would really help me get into law despite finding it difficult.. I ended up getting a C at AS. Luckily I chose my other three subjects based on what I am best at so I have not had a problem with getting offers.