Unfortunately the victims won't report it to the police, so I'm afraid you will have to take what I say on face value. 
The OP did describe the working girl in question as "a stunner" so if he revisited her with the cash he may very well have a different experience, hopefully positive only one way to find out!
Ive never met the girl in question and ive just left a neutral review so "White Knight"? I don't think so.
I believe that the half baked laws regarding prostitution in this country do leave girls in very vulnerable situations and unfortunately it gets exploited.
I take nothing you say on face value, what makes you think this lass has been subjected to the robberies you claim.

The OP walked because of her attitude, not because of her looks and as he says what kind of a punt would he have if the attitude is all wrong, yet you are trying to say a bad attitude is justified.
If you have any suspicions of anything untoward, be that them being robbed, trafficked or coerced report it to the relevant authorities, don't post your speculation, guesses and opinions on here as fact.
Tell you what just say and I'll find out how you can contact the Northumbria police dedicated liaison officers who will take your accusations very seriously along with treating you with discretion.