I agree with the person above on using this time to gauge your passion for law, but I don't believe that you should do it as an A level unless you're planning on taking four. Many unis will probably prefer you to not do law at A level due to the methods in which you practise it not being as advanced or technical as the LLB standard, and although you'll understand how many areas of law you cover, it'll be like having to re-walk the path you've trodden on, by what I've heard. I would ideally (coming from someone who is hoping to get into law through a conversion course) choose essay-based subjects that prove analytical skills such as English Literature, and one that analyses and picks apart certain events such as History (for past events and how they affect the present, same as cases in law) or Politics (for political milestones and manifestos that have shaped the country/world into the form it is today). Politics does normally cover the basics of the UK judicial system so you do get a small insight through that too. I'd also look at how they are assessed, as English Lit is mostly essays with 1-2 coursework bits and Politics is normally straight essays if I'm not mistaken. I don't know enough on History to comment. If you are able to take a modern language, I would highly advise on that, too.
Gauge your passion for law elsewhere. It's a highly competitive field everywhere (but especially if you are aspiring to a RG for law) so use your extracurriculars and holidays to network, get experience, talk to current law students etc and that should make you well-rounded enough to get in (and also allow you to change your mind if that happens two years down the line). Try debating, journalism, summer schemes, online discussions, and read.