I applied last year for Electronic Engineering and had an interview, so I was in the same position as you.
The day basically consists of listening to a few talks about the course, accommodation, stuff like that, then you get a free lunch, and then you'll go to your interview in the afternoon.
I personally wore a suit for the day, but I like to be professional and dress smart, even if it says smart/casual, I feel it helps to set a good impression.
Remember to smile, to greet your interviewer, to ask them how their day was, etc, just be friendly. The questions they'll ask will mostly be of the type, "Why did you choose this course", "Why did you choose this university", "What do you feel you want to do in the future", and things like that. Try to be enthusiastic with your answers, expand whenever possible. Be honest. If they ask you why you chose the course, and you answered that you were mainly in it for the money, say so! It'll probably be amusing for them
By way of technical questions, I was quite surprised to find out I only had one simple maths differentiation to do, which my interviewer just wrote down on a piece of paper and asked me to do. That's it. However, there has been a rise in the number of applicants every year, so I really would do a little revision on maths or physics topics, in case it's more difficult this year.
I'm now about to finish my first semester at Surrey, doing Electronic Engineering.
Hope this helps