The police may well have a couple of people devoted to this kind of activity, running it alongside their anti-trafficking team. A British underage prossie is as much concern as a foreign trafficked one, and will create the same interest.
My guess is that the couple the OP writes about have come to police attention, and someone has been arrested. The police have got access to the AW account and are talking to the contacts on there one by one. If the OP never met them then he's done nothing wrong. Police will be looking to build up a picture of how the girl was offered: how was communication carried out, were any phone calls made or texts, if so what were the numbers? How old did the girl claim to be (AW said 18, but was anything else said verbally? Was as address given? What was the price? What services were you offered? Why did you pull out - did you sense something wrong?
The police will be asking this of as many people as possible to try and get a feel for how the business was wrong: with these kind of prosecutions they like to cross every I and dot every T.....as much info as they can get.
Remember that they're probably hoping the OP will talk to them because he (by his story) has nothing to hide: any bloke who did fuck the girl is probably going to end up in shit street and if he has half a brain will say nothing