Sounds like a good experience!
I think the key to doing well in a law interview is to stay calm, and look very carefully at the materials you are given (if any). Take your time to think your answer through, and if you're stuck, try to think out loud because at least the tutors can see your train of thought. It really doesn't matter at all that you have no legal knowledge - what the tutors want to see is how you think, and how you respond to and apply new knowledge.
Law interviews seem to have varying formats across colleges, but you might want to try reading a cases like Rose v Plenty (
http://www.bailii.org/ew/cases/EWCA/Civ/1975/5.html).
After you've read the case, try to answer the following questions:
1) In no more than 25 words, summarize the facts of the case.
2) Who were the judges sitting in the case? Which party did they rule in favour of?
3) What was the rationale behind the judges' ruling?
4) Let's say I'm a security guard at a restricted area. While I was working, I saw someone trying to climb over a perimeter fence, and I tried to stop them. In the process, he accidentally fell and broke his arm. Should my employer be liable? Why or why not?
5) Same facts as above, except that this time, my employer has a strict no physical contact policy. The man fell because he got a shock when I grabbed him. Should my employer be liable? Why or why not?
6) Same facts as above, except that on my way to work, I ran into someone I hated, and I hit him such that he fell and broke their arm. Should the club be liable? Why or why not?
You can also try reading a statute, for instance the Offences Against the Person Act 1861 (
http://www.legislation.gov.uk/ukpga/Vict/24-25/100/contents). Look at sections 20 and 47. How would you distinguish between what a wound, actual bodily harm, and grievous bodily harm?