i made some notes on the sort of questions i thought might come up and what sort of answers i would give (many of which did come up) in my Westminster interview last year, and it appeared that i went down well with them as i got a place. pm me if you want, and i can send them to you.
I would say dress smartly - do you have a jacket? and then a blouse and trousers or skirt and tights or something... you are probably better placed to give female fashion advice than me....
At the interviews for the sixth form at my school this year, you saw girls turning up in hoodies and uggs with really slutty skirts, etc.... that sort of thing doesn't go down well. I was talking to the head, and some of the things he said they observed was just how you behave whilst you there. Again, at my school, the candidates would wait and chill in the common room between interviews where there was food and drink, etc. And the senior teachers would come in from time to time and see if anyone was behaving boistrously or anything - so just be sensible. another example was they had provided newspapers and magazines, and this group of people were sitting at a table and a load of the magazines were on the floor besides them. The headmaster then came in and bent down to pick them up and was tidying them up for 2 - 3 mins or so, and they did not hesitate in their conversation let along help him. so they were scored off the list as far as i am aware... to have the headmaster grovelling at your feet and not help... I am sure that won't be a problem though.
Umm, be relaxed in your interviews and they are always pleasant. They are there to get the best out of you and they aren't trying to catch you out or trick you or anything. Remember, always think before you speak. It is fine to take a moment to think before beginning an answer to a question.
I can give you some more tips on interviews and things (i quite enjoy them really...) so ask any questions if you have some and i'll get back to you tomorrow (it is too late now.... i'm tired)
good luck!