Paying for sex is only illegal in Northern Ireland.
In the rest of the UK it perfectly legal. However most activity around it is illegal such as a brothel keeping, pimping, trafficking, soliciting, kerb crawling. Ads state “only paying for time” to get round the soliciting law.
By the way if you encounter a trafficked sex slave or a child, you have no defence in law, regardless of what the pimp or advert told you. Do not pass go, do not collect £200.