S.51 A Sexual Offences Act 2003
(1)It is an offence for a person in a street or public place to solicit another (B) for the purpose of obtaining B's sexual services as a prostitute.
(2)The reference to a person in a street or public place includes a person in a vehicle in a street or public place.
(3)A person guilty of an offence under this section is liable on summary conviction to a fine not exceeding level 3 on the standard scale.
(4)In this section “street” has the meaning given by section 1(4) of the Street Offences Act 1959.
The picture shows prostitutes cards often referred to as 'Tarts Cards' a common sight in phone boxes in major cities across the UK.