I worked in Soho for too many years to know a scam when I hear one.
#1 Prossies who are sitting and doing nothing are not earning money for either themselves or in this instance their pimps.
#2 OP states in first message that there were 12 sitting and 6 standing on the stairs, then it becomes 12 standing and another 12 hanging about. Go back to #1 and thats 24 (or even 18) prossies doing absolutely fuck all and not earning !
#3 Not even with a regular will a prossie be happy about you walking out without earning a bean from you and none allow freeview !
#4 You say he picked you out of a crowd, really, that's clever of him to be able to spot you as a punter?
#5 18 or 24 prossies plus pimps and 2 punters is alot of people in one place, you said it was a large georgian town house, I know Soho like the back of my (right) hand and a private residence that isn't an office narrows it even further so this can be only on a couple of streets, so to keep it quiet without the rozzers turning up makes me even more dubious about its existence for a true punt - something just does not add up - if it looks to good to be true, and in this case it just sounds to good to be true, then you know what, it most probably is too good to be true..