After thinking a bit, I support the UK government's new proposal to prosecute men using "forced" prostitutes.
Yeah, it's hard to prove one way or the other whether someone is "forced" eg. by circumstance such as drug addiction. And it raises interesting questions about responsibility and agency (ie. if a prostitute can't consent to sex if she's motivated to pay off her addiction, why is a burgler responsible for a robbery if he's motivated to pay off his addiction?)
But nevertheless "forced" is where you'd like to draw the line. All the cases where prostitution is justified will fall on the "unforced" side. Ignorance is already no defence if, say, you're receiving stolen goods, so why should it be different if you're having sex with enslaved women?
Of course, I think that the corollary is that brothels where women are "unforced" should be legal. (Which would further protect sex-workers.) This shouldn't be seen as a step towards the total ban on prostitution.
And personally, as with other currently criminal activities (such as supplying heroin) I believe that brothels should be legalized but only as non-profit organizations or workers' co-operatives. In other words they should be legal and allowed to provide income for their workers but not provide profits to non-participating shareholders or include too much structural incentive to "grow" the market.