As far as I can tell the advertising is only illegal if it is in what constitutes a "public place". IANAL but I doubt that a privately owned website would count as such and if it is illegal it doesn't appear to be enforced at all.

Women who work as prostitutes will usually pay taxes as self employed people and can actually claim advertising (as well as various other things that are best left to the imagination) back as business expenses.

A privately owned website that is generally available to the public most certainly does constitute a public place (in the same way that standing in a pub saying "who fancies a shag for £50" wouldn't get you off the hook. If you're soliciting TO the public, it doesn't matter where you are "standing".

Escorts pay tax and deduct expenses for the "Escorting" work they do, not the sex. If they declare the income as income for sex, the police quickly come knocking to arrest you for "living off immoral earnings" (a few have done so in the past as a "protest").

It is only an offence to live off the immoral earnings of others, not your own.

“Public place” includes any highway and any other premises or place to which at the material time the public have or are permitted to havea ccess, whether on payment or otherwise ”.

I wouldn't think a website constitutes a "place" under such a definition. Otherwise posting a picture of your genitals on a website could be viewed as indecent exposure, though IANAL.

Yeah, one sex worker rights activists described the approach politicians take to prostitution as "they don't criminalise us, they just criminalise everyone around us". So if a woman dares to have sex for money whilst living with a partner or with a teenage kid over about 12-14, their partner or kid can be arrested for living on immoral earnings. (Some other countries are even worse; apparently in Sweden landlords risk being arrested if they don't kick out sex workers renting from them upon discovering their line of work because it's illegal to make money from them full stop.)