ive got a uca applicant day tomorrow! i know somebody who went last year, and from what they told me, you leave your portfolio on a table so the staff can have a look at it, then you get called in to discuss it. when they say the meetings are informal, they mean informal. a very casual conversation, mostly about you and your interests in relation to the course. they might ask you about what you think the strengths and weaknesses of your portfolio are, to point out and briefly discuss your favourite piece, or about the course you're currently doing (or was prior) etc. but their applicant days are more about them getting to know the person behind the portfolio, rather than critically analysing your skill. it's all done as if you were just having a casual chat.
my friend also said that the portfolio reviews were done in pairs, so two people would be called in together, but this might not always be the case.
as for the day overall, from what i know, you register when you first arrive, have a welcome chat, a tour of the campus, looking at the department for your subject, stuff like that.
obviously, things might be slightly different for you, but overall you should just keep in mind that it's not a job interview. it's not super formal and professional. they're just trying to get an idea of who you are, and why you want a place on the course. try not to stress about it too much.