As PPs indicate, money for sex services isn't illegal/criminal in the UK but there are some restrictions.
The exchange of sexual services for money is legal in the UK (apart from in Northern Ireland where it's illegal to pay for sex).
But, these related activities are illegal under the Sexual Offences Act 2003:
- soliciting (trying to get clients) on the street or other public place, including someone in a vehicle
- paying for the services of a sex worker who's forced or threatened into it
- owning or managing a brothel (any premises which is used by more than one person for sex work)
- pimping (someone who has control over sex workers and the money they earn)
- advertising sexual services, including putting cards in phone boxes
https://www.cityoflondon.police.uk/advice/advice-and-information/sw/sex-worker-safety/#:~:text=The%20exchange%20of%20sexual%20services,including%20someone%20in%20a%20vehicle
For solicitation, I don't know about convictions and sentencing but, according to Statista:
There were 184 offences of soliciting for prostitution recorded in England and Wales in 2023/24, with the number in this year the fewest offences for this type of crime recorded in the provided time period. Compared with 2002/03 when there were 2,111 offences recorded, this type of crime has fallen substantially in recent years.
https://www.statista.com/statistics/283114/recorded-soliciting-for-prostitution-offences-in-england-and-wales-uk/