If prostitution is legal in Britain, but advertising sexual services, streetwalking, brothels and kerb crawling -- driving slowly to ask women for sex -- are all against the law. How could you be a prostitute legally? Is it just a case of if a man apporaches you and offers to pay you for sex you can say yes?
I think you advertise your services as a masseur, give a massage and collect payment. The sex afterwards is free, apparently. It is the typically British, nudge nudge-wink wink scenario, I believe. I may be wrong because I'm an innocent sort really, honest!
I think suffragette is probably right. Many prostitutes have a good client base and just see regulars in their own homes. As they don't street walk, don't solicit and don't advertise, they are doing nothing illegal - apart perhaps from tax evasion since I doubt very much their earnings are declared.
You can sell sex in a building, such as your own home, provided there is no more than one woman working. (Subject to council restrictions re running a business from home).
If a man approaches and asks for sex, he could be arrested for harassment if the woman is not a prostitute.
In London there are a lot of walk up brothels - so called because you walk up stairs to them. One girl in a room, working on her own, and sign on the door. Perfectly legal.