The most important thing is that you get the grades that they require. This will vary, but for the most part law at an RG uni is going to ask for A*AA-AAB.
There also tends to be a preference for traditional and rigorous subjects. Classic A Level choices for law include English Lit, History, Government & Politics, RS, Philosophy and MFLs, though science and maths subjects are usually acceptable (though I'd still recommend that you do at least one essay based subject, so as to develop your essay writing skills). Note that law is often regarded as a soft subject at A Level, and though doing it wouldn't prevent you from doing law at an RG uni, it's not really going to count in your favour and may actually be a disadvantage (counter-intuitive, I know!)