I am sure Lady Bellatrix can give you much more useful advice that I can but I did session in two different dungeons while there. One was West End Dungeon http://westend-dungeon.co.uk/ which is rented by the hour just like here in the U.S. I think it was 50 pounds an hour. The other was Better than a Bed which was a bed and breakfast type dungeon. http://www.betterthanabed.com/ I rented that by the day and also stayed there. The proprietress was a hoot to hang out with.

Both were very conveniently located. I was advised to stay in Central London and I believe they were both in W1. Both were very well equipped, but Better than a bed was more atmospheric.

I made contact with these dungeons well ahead of time and asked all the pertinent questions. A colleague of mine got a nasty surprise at a dungeon there recently when she had to pay up front to book. Yikes.