hey.
im a first year natsci at ucl.
hopefully this isnt too late and you havent had your interview yet.
when i had my interview last year, it was quite informal.
i cant promise yours will be the same, but i should imagine it would be.
they split us into two groups, one did an interview while the other had a brief tour, then they switched round.
the interview was pretty easy.
they asked which area i wanted to be interviewed on (phys chem bio) a couple weeks before via mail, and then on the day there was a 1 on 1 interview with a lecturer from chosen dept.
all the 1 on 1 interviews take place at the same time in the same room, which helps you feel a bit more comfortable as your not so alone.
it was essentially just a casual chat about your subject, why you like it, and your personal statement.
the whole thing was over by lunch time, and according to the 2nd years at the time, it was essentially just a formality - you would get an offer as long as you were at least reasonably sane.
personally id take that with a pinch of salt, but as long as you know your personal statement and have something to talk about youll be fine.
you get a free mini buffet as well - the juice is good
also - wrt the IB - they may not prefer it, but i do know 3 peeps in my year who got in who did IB, so at the very least they dont totally reject it.
hope this has been some help. gimme a shout if you have any other questions, and gl with your interview.