Many law degrees, although they do not explicitly state a preference, do prefer essay-based subjects and subjects which require a lot of critical thinking and verbal reasoning. The best ones for this are usually English Literature, History, Politics, MFLs, Classics, etc. These are just a general guide, and most universities will not expect three of these subjects, though having at least one of these, especially English Literature and History, will give you the same sort of skills that would be expected of a law degree. Maths and other sciences are very useful in terms of problem-solving and analytical skills, which will be very pertinent to your degree, but a more competitive application will have at least one essay-based subject, if not more. What may be of particular use to you is pursuing an EPQ, looking into an area of Law and showing further commitment to the subject, as well as joining debating societies and other, related extra-curriculars and work experience. It is generally not advised to take A-Level Law as many universities prefer to take a 'blank-slate' approach, and they are aware that not every student will have access to this more niche subject.