In the punters defence, how would they know?
The laws inducted circa 2003 by the last Labour Government are so draconian, the law aforementioned is just one hideous sanction bringing Latin Law into the UK for the first time, it is the same with the age of consent, an example is a lad who legally has sex with his 16 or 17 year old girlfriend is deemed a paedophile if he has photo of HIS girlfriend sitting topless on holiday, the law contradicts itself.
To address your point "how would the punter know" it is presumed guilt, as written - Latin Law - guilty until proven innocent which is virually impossible and a possibility that one could end up on the Sex Offenders Register.
There needs to be root and branch reform of UK Sexual Services, legalisation of brothels, sanctions on rip off online services and clarity of the age of consent with upper age limits like Europe.