Ours accept visit but only with first dc. Which I disagree with as was very ill with first and couldnt remember anything, I was scared also to go back and think it would of helped me. I went at about 35/36 weeks which was a good time to go.
I went at 32 weeks - they didn't accept people before 30 weeks. However, I had to book weeks in advance as the places go quickly - so whilst you are wondering about when to go, I would give them a call to make sure you'll be able to get a space at all!!