You'll be fine I went to the chemistry open days last year (I changed to maths later), and they gave you really clear instructions and had people to guide you through it all. I found Sussex had the best freebies such as a mini calculator!
Been there - don't worry about it! Just have fun. Everyone, even the most experienced will be worried about that. If everything goes okay, you don't slip up, and you decide to go to Sussex, then you won't be remembered for the admissions day, and you can make friends anew when you start.
If you slip up and do something embarassing, just don't get upset. It will be a little anecdote the other people can tell when they get home. Most people won't remember who it was when they start, any that do will just be instant friends when you start, and you'll have a fun little story to tell people when you start. "Eek, those practicals on admissions days -- I mixed the wrong the wrong thing... look! You can still see the beaker embedded in the ceiling!" is so much more interesting than, "So, what A-Levels did you do?" "Chemistry, Physics, Maths." over and over.