Seems there is no vice squad any more or whatever old bill dealt with it.
They aren't called that as mentioned but they are still active with purges designed to nick kerb crawlers, which of course is the easiest thing to do if they choose to do it. They know the where, when, why and sometimes who, in my view its utter madness to pick street WGs up. It could lead to court of course, your name in the paper or in some areas your face plastered on the police website, and even the seizure of your car, I assume that's for persistant offenders.
Plus you could get clipped by just the WG, her and her pimp, be pulled over by the police only to find the WG has class A on her, crack or heroin which could cause you bother dependent on how the police feel on the night, or pick up an STI as street WGs aren't exactly known for being the cleanest of WGs.
For not much more a London punter could go for a quickie in a Soho Walk-Up with little chance of police bother.