That's not strictly true, to be guilty of trafficking within the UK the trafficking has to result in an offence under part 1 of the sexual offences act, so prossies being driven around willingly would not result in an offence.
theres new catch-all legislation, which is what is now being used: Section 2 Modern Slavery Act 2015
External Link/Members Only"Arranging or facilitating the travel of another person with a view to exploitation - Section 2 Modern Slavery Act 2015
A person commits an offence if the person arranges or facilitates the travel of another person (V) with a view to V being exploited.
It is irrelevant whether the victim consents to the travel (whether V is an adult or child).
A person may in particular arrange or facilitate V's travel by recruiting V, transporting or transferring V, harbouring or receiving V, or transferring or exchanging control over V.
A person arranges or a person arranges or facilitates V's travel with a view to V being exploited only if
the person intends to exploit V in any part of the world during or after travel; or
the person knows or ought to know that another person is likely to exploit V in any part of the world during or after travel.
Travel is defined as:
Arriving in, or entering, any country
Departing from any country, or
Travelling within any country.
A person who is a United Kingdom (UK) national commits an offence regardless of where the arranging or facilitating takes place, or where the travel takes place.
A person who is not a UK national commits an offence if any part of the arranging or facilitating takes place in the UK, or the travel consists of arrival or entry into, departure from, or travel within the UK.
Section 3 defines the meaning of exploitation for the purposes of section 2. A person is exploited only if one or more of the following apply:
Slavery servitude and forced or compulsory labour, where a person is the victim of an offence under section 1 Modern Slavery Act 2015 (see Slavery, Servitude, Forced and Compulsory Labour below).
Sexual exploitation, which involves the commission of an offence under
Section 1(1)(a) of the Protection of Children's Act 1978 (indecent photographs of children), or
Part 1 Sexual Offences Act 2003
which would involve the commission of such an offence if it were done in England and Wales."
theres more, but those are the key parts