If the maid offers you the chance to wait it's worth taking, as it normally means the girl's got 10 minutes or less left with the current client, and normally there's a tv on that you can watch. You won't see the other guy either, a lot of walk up's have curtains they'll draw across, or in the case of Anna's address, a separate door to let the guy out of.
I used to have your concerns, but after almost 20 years of on and off punting in Soho I don't care anymore. Mainly because no one else does. And the chances of bumping in to someone you know are incredibly low.
Prices at the mo are (roughly) £20 for sex, £25 for oral and sex, £30 for oral, sex and positions, and between £45 - £50 for 20 minutes. It varies slightly from walk up to walk up, but I'd never pay more than the above.