What do you mean, I thought prossies always pay full tax on their earnings. I would be shocked if this isn't the case.
Any money they make on AdultWork from private gallery sales, webcam sessions, etc (which a lot of WGs make money from in addition to their real-life punter bookings) is paid into a bank account with "AdultWork" clearly shown as the payer. So for these things they would be paying tax at least since it would be difficult to hide such earnings from HMRC. Of course the majority of their earnings is all off the books and paid in cash. If you were self-employed and every customer you had was happy to pay you in cash, would you declare all your income?
So they're probably not paying full income tax on their other earnings but at least we can rest assured that they'll still be paying a significant amount of tax anyway, considering the amount that the stereotypical WG spends on fags and booze, lube, blue radox, etc, etc