Soho is a busy part of London, and people will see you walking in and out. On the positive side, none of them will care about what you're doing. There are also more walk-ups in Shepherd Market in Mayfair, which isn't as busy as Soho.
If you need 30 minutes, you'll have to spend £60 or more, and there are much better options to be had for that money as services on offer will be limited (very much doubt you'll get DFK). I think you'd be crazy to spend an hour in one of those places, so walk-ups may not be for you.
The doorways are all pretty close together, so it won't take too much walking around to find someone you like the look of.