I believe 'carding' is the offence.
(Criminal Justice & Police Act 2001, S46)
'Placing of advertisement relating to prostitution...in a public place.'
Being the 'mouth-watering siren', referred to on the card, is not an offence.
Unfortunately, still waiting for hunky coppers to knock on my door... mmm !

Card-boys take prosecution on the chin...pay the fines, no big deal.
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Tart card started out as discreet postcard sized ads in newsagents' windows, advertising along the lines of "large mahogany chest for sale", "French and Greek lessons","A-level tuition" and such like. Unoffensive to kids, tourists, and country bumpkins of a nervous disposition. Then the police went round the newsagent in an attempt to put a stop to the ads, threatening with prosecutions for 'living off immoral earnings', which was still an offense in those days, or simply threatened to report them to the Inland Revenue. Tarts maids would have to go around London once a month, and pay for the ads for the next 4 weeks upfront, in cash. Some paper shops had as many as 100 - 120 cards in the window, a tidy tax free income.
So the shops would no longer take the cards, and a new way of advertising had to be found. Stickers on parking meters were trialled and dismissed. Men couldn't discreetly remove the sticker or make a note of the number. Then somebody hit on phone boxes. Once the guys were inside, they could simply pick up the phone, and start dialling, and it was not immediately obvious what they were doing. That was before the mobile phone, of course. Once everybody had a mobile, there were only two reasons for a man to be in a phone box: to pee or to phone a prostitute.
There was a lot of competition, so some bright spark thought of using pictures instead of just text, and that's when the whole thing became offensive to some. Having tits and ass at eye level all over Central London was not everybody's ideal. All kinds of people started to remove them: feminists, cops, BT employees, the fledgling priests of the Brompton Oratory before Sunday Mass, little old ladies, the dirty Mac brigade who collected the things - ah, happy days!
I'm often tempted to start carding again, writing only, no pictures, like in the olden days. Quite a few parlours in Central London are carding again, but it's not so much of a problem any more, since a lot of phone boxes have been removed, so there are not many left. The only thing to be careful of is not to have them on the premises. Once you start carding, the police will come eventually and search your premises, so always collect the cards from the printers and hand them straight over to the carders - never keep the boxes in the kitchen cupboard.
Getting waaay off topic. Real rubbish weekend. New Vivastreet ad out, and I had a huge call volume of complete numpties, and not a single booking from them. Better luck next week, when the normal guys come out to play again.