Just to add my 3c
Make the appointment online  - it will be for a few weeks in advance. Go for a morning one and get there early. As long as you have an appointment, they will see you. Do not forget to take a print out of the appointment with you - they will ask to see it.

Mine was for 10.00am, I went in at 9.30 and was out again in 20 minutes.
if you make it later - there are huge queues - I think the appointment times actually mean nothing, and the later in the day you are, the more of a back log there is.

HTH