JAM, welcome to pleasure dome.
If you have ever visited Amsterdam - London is a virtual version of the street windows in that you never know who is actually available until you call them on the day. I suggest you (1) identify a long list of potential visits (10 at least) and keep an eye on the days and hours they work from several days in advance (2) call your likely available short list (at least 4) the day before to ask if they are working, (3) call at the time suggested by the girl's on the morning, (4) go with the first who answers and books, (6) text to let them know you are leaving and arrive in good time, (7) have a HoD back up.
You won't be sure who you will get till the day but you at least stand a good chance of getting a fuck from your preferred long list. Don't rely on green lights - call then text.