Should be straight forward and the advantage you have is an appointment slot.
Before your appointment, take your paperwork including appointment confirmation straight to the booking counter inside, to the right of the information counter. No need to get any queue number from outside.
From there they will take your paperwork and ask you to be seated somewhere near counters 7 and 8.
You will then be called for a photo and anything else they may require.
Whilst you may not be in and out according to your 20-minute time slot, even if busy, you should be finished inside an hour.
As you are extending, a reminder your bank letter will need to be of a 12-month statement type.
I recently did mine with an appointment and have never seen so many people there, yet everything ran smoothly, the staff I spoke with were all helpful, I was in and out in 50 minutes, which is no big deal for something once a year.