Yeah, I know what you mean about Oxford. It's closer to home for me too, and I didn't really know Cambridge. However, I really loved Cambridge after that conference, and pretty much decided that was where I wanted to go. Same also about law - I wasn't completely sure on that either about this time last year - it's difficult to decide when you've never studied a subject before. You'll find the uni and course that really suits you anyway - there's usually something that just feels right about them. Definitely try to go on one or two college open days if you can, when you've made up your mind where you'd rather go, and which subject. They are really worthwhile, and you get a chance to talk to directors of study for your subject, and get a feel for whether the place suits you. Particularly good if you get the chance to stay overnight.
I don't think it matters greatly about reading law books beforehand. I remember reading some of 'Glanville William's Learning the Law' before I went on the conference, but while parts of it are good for written explanations, it's a little hard-going and you probably won't pick up a lot to begin with, as it is all new to you. It certainly didn't make any difference. On the conference they expect you to know little or nothing about law, so don't worry too much about it. The lecturers will explain everything a lot clearer, and so long as you learn a little there, you should get a fair idea of whether law is right for you.
If you want to read a bit around the subject I think Helena Kennedy's books 'Eve Was Framed' and 'Just Law' are a little more interesting, giving real examples. Except, I don't know whether it was just me, but I found parts of them a little over-the-top in terms of almost assuming that everybody is innocent. It's hard to explain; while both books contained case-by-case examples, showing individual miscarriages of justice, there were quite a few general statistics that made it seem like all the people in that group had been wrongfully accused. I don't know, as I'm bound to come off wrong here, so I will admit they are both very interesting and shocking books which open up your eyes to the injustices still present today. However, with my scepticism, I also found them good interview practice, as I tried, in my head, to challenge what I thought was a little sketchy or inaccurate, and thought what more information I would like to know. This, I think, was far more worthwhile than reading lots of books explaining the divisions of courts and roles of barristers and solicitors - as valuable as they are, this information can be explained to you at the conference, open days or if you do work experience, and I'm not convinced I fully digested most of the sketchy overviews of law I read from books. Another good interview practice is to read articles in the paper, especially if you can find a strong link between the topic and law, and see if you agree with the journalist's stance, or any quotes taken from officials/experts. See if you can argue either their point of view, or against them (and also think of their possible counter-arguments). (This will come in handy for French speaking as well!!!)
Anyway these are just suggestions - if I'm honest I didn't spend a lot of time doing either of them, but what I did try helped. Don't worry if you find some aspects a little dull - law covers such a wide area, there are bound to be bits you enjoy more than others - just focus on what you do enjoy.
I hope you have fun on the conference - even if you decide law isn't for you it's a really good experience! It is horribly competitive, but if you enjoy it go for it, and really enjoy every second! I was so scared about the competition and the interview and everything, but luckily it all worked out for me, and, three As permitting, I'll be off to Cambridge in October! You've got fantastic GCSE grades, and so long as you work hard you and really go for whichever subject you choose you'll have no problem!