Well, simply say: Prostitution is legal in the U.K. Soliciting, pimping and running a brothel is illegal.
In the United Kingdom, prostitution itself (the exchange of sexual services for money) is legal, but a number of related activities, including soliciting in a public place, kerb crawling, owning or managing a brothel, pimping and pandering, are crimes.
In England, Wales and in Northern Ireland it is an offence to pay for sex with a prostitute who has been “subjected to force” and this is a strict liability offence (clients can be prosecuted even if they did not know the prostitute was forced).
It is illegal to buy sex from a person younger than 18, although the age of consent for non-commercial sex is 16.
Quite sensibly the U.K. government recognises that criminalising payment for sex would not survive proper scrutiny. For example is accepting a meal in a restaurant a payment? a room in a hotel? a piece of jewelry? a vacation?, a flat? money for a dress?. The police would end up having to investigate every mistress and courting couple in the country.
Prostitution associated with escort agencies and massage parlours is tolerated by police provided there is no other criminal activity involved.
Streetwalking is “soliciting” and “kerb crawling” to pick up prostitutes is a specific criminal offence. Despite this some police authorities turn a blind eye to it, where it is restricted to non-residential areas, such as industrial estates at night. Street prostitutes know that the police have insufficient resources to keep arresting them, so there is always going to be a compromise.