It is quite simple, study subjects that you will do well in. For law, you can study any A levels you like as long as they are the traditional/academic subjects like history, politics, geography, sciences, maths, English, economics, philosophy, languages, sociology, psychology etc etc.
At the end of the day, what is important is the grades you get. Unless you are good at maths, I would even suggest you do not do this because it is one of the harder ones and certainly not required for law.