I'm not sure this is true. A huge amount of sex work in the UK is conducted through Internet sites now through which sex workers advertise and screen their clients - who are essentially ranked/reviewed, with any problems or difficulties flagged up. People can choose not to accept clients who don't satisfy certain feedback criteria or who don't have enough feedback. The Swedish model (criminalisation of clients) makes this system impossible, as clients would not be willing to register for such a service, making screening much more difficult. The obvious response to policies like this which affect safety are for sex workers to work together in groups - but unfortunately as Tea has mentioned, a group of two or more sex workers now legally constitutes 'brothel keeping' and is also illegal.
OK, on the face of it this seems like a decent argument against the Swedish model, but on the other hand I can think of about a thousand ways offhand to game that system from both sides.
Can you say with any conviction that someone who wishes to buy sex in the UK online or otherwise has any way to be certain the woman involved is not being coerced, abused or exploited?
Is there a source you trust that indicates the level of trafficking and/or vulnerable women forced or coerced into prostitution in the UK?