Don't worry - there's other Open Days throughout the year, so if worst comes to worst, make sure you're booked for those.
You can also just turn up at the University and look around, although obviously it won't be geared up to showing off the Uni.
As far as I can remember, I wasn't asked to sign in when I went down, although that may be because I got a friend studying there to show me around. Of all my Open Days, I was only asked to sign in at Sheffield, and even then I think there was an option to register if you hadn't booked online.
The main reason they ask you to book is so they have an idea of how many people are coming, and how many supplies they'll need.