Greetings,
If you are very keen on reading up on Law before the start of Univ. I would recommend M. Zander`s, English Legal System. Although, it is a little advanced, the depth is not likely to be required of you, perhaps even at Univ. However, its an excellent introductory text.
If you find that it's likely to take up too much of your time, read an alternative on the ELS, Slapper and Kelly is pretty good. Also, if you have even more time on your hands, may want to read something on Contract or Tort, those will be very important to you later on.
Alternatively, if there is any particular branch of law you are interested in, then, take a look at a textbook designed to cover it. I'd use the library, rather than purchase, aside from the Zander book, possibly. Esp. as law text books can be very expensive, well, like much else! If interested, write back, I'll give you more detailed advice. Although, I wouldn't have thought an A level student was likely to have so much time on their hands!
J.S.