Well my test centre was just a small and slightly depressing waiting room with 2 computer rooms to the left and right
Took it a while ago now but from what I vaguely remember, once you get in tell them what time you booked for, then hand in your license + counterpart, sign on the little touch screen device, then they'll get you to empty your pockets and put your stuff in a locker
They'll call you to go in one of the computer rooms. The person who checks your provisional before you enter will explain everything pretty much so there's nothing really complicated about the process. Just wait your turn, once you get in, do the test, take the practice questions if necessary, but I didn't
If you finish in 15 mins, just go over your answers then move on to the hazard perception. That takes a little while longer than the multiple choice questions because the clips are pretty slow and you go through each one twice. Like I said, nothing majorly complicated. Once I finished I headed straight out and got my results within the next minute