This discussion is getting towards the point of being off topic, but...
'Control for gain' is the offence, not 'promoting selling sex'. The gain bit is obvious: agencies make money off other people's prostitution. Control doesn't have to involve coercion or trafficking or anything else that antis go on about, but includes telling someone where an appointment is. (Proof: they wouldn't have gone there if you hadn't told them, and if an agency couldn't arrange for a prostitute to turn up, no-one would call them.)
Is anyone going to believe that agencies do not know that what they're doing involves prostitution? No, and that includes the police, the Crown Prosecution Service, and the people who sit on juries. I once had an agency owner in trouble with the law on the phone saying '.. but I say it's for "time and companionship" only..' and pointed out to them that their website also included links to reviews that made it very clear that they knew it what was happening.
Why are there so many agencies out there? Because the police have better things to do than close all of them. If the owners behave in other ways, like paying taxes on the income, they will probably get away with it. Treating your staff well helps too: happy staff are much less likely to go to the police and, if the police turn up anyway, less likely to spill all the beans.
Most agencies die before the police get around to dealing with them because they're crap at the business end and/or don't do enough for their escorts for them to want to stick around paying them a cut.