There's some shared rooms in some halls - very, very few, though. It says in the accommodation prospectus which halls have them. I think there's about five shared rooms out of 400+ in Badock, for example.
If you put down that you want to be in a single room, there's a 99% chance you'll be put in a single room. If the worst happens and you get a shared room, someone else in your hall will move out after a couple of weeks, get your roommate to move into the new place, and you get a bigger (and cheaper) room to yourself!
But seriously, I wouldn't worry about it.