I didn't know either so went with a full ish bladder. I asked at reception and they said I could go to the loo. I did get told off at my 12 week for having an empty bladder but the letter didn't mention it and there were no signs in the loo!
I just drank a cup of tea half an hour before, was fine. The full bladder for 12 weeks is because the uterus is still within your pelvis and the bladder pushes it out to see it easier, at 20 weeks your uterus is about up to your belly button so much easier to see.