You don't want any of these to catch you by surprise - and they're bound to come up.
1. Why this firm? They may seem like an undifferentiated mass to you, but each firm sees itself as unique. Ahead of interview you'll need to do a bit of digging as to how the firm sees itself, its market position and its future. Chambers Student "True Picture" guides are your friend here.
2. Tell us about yourself. Yikes. Hopefully your interviewer will be more subtle, but be prepared to make an elevator pitch about who you are and what your background is. No "ums" - be prepared and sound confident.
3. What makes you a good fit for this firm? Like a Frankenstein's monster of questions 1 and 2, this one requires you to show off your knowledge of the firm, and point out what makes you a good candidate. Easier to answer if you've done a vacation scheme at the firm, otherwise you'd do well to focus on two/three characteristics of the firm, and why these appeal to you. (Make sure it's something substantive - a Starbucks in the lobby doesn't cut it.)
4. What made you decide to be a lawyer? Another classic question. Firms want to know that you're in it for the long haul, and have thought through your choice of career. Focus on how aspects of your personality fit the skill set that lawyers need.
5. Why should we choose you? A test of British reserve, this question requires candidates to walk the fine line between confidence and arrogance. You should assert that your skills and experience make you the best person for the job - it's even fine, if you think the interview has gone well, to turn the question around on the interviewers with, "Do you have any doubts about my suitability, that I can clarify for you?" Avoid getting flustered or embarrassed, and take the opportunity to sell yourself.