It is only illegal to live off somebody else's immoral earnings (i.e pimping / running a brothel). It's also illegal to solicit in a public place (i.e kerb crawling).

It's certainly not illegal to respond to an advert offering paid sex or to place such an advert. I have no idea what the law would be regards propositioning people at random.

Edit: Paying for the services of someone who is coerced into prostitution against their will is also illegal.

Source: http://sw5.info/law/punters/

Immoral earnings? Based on who'se morals? I hope it's not actually worded that way.

Here's the act (http://www.legislation.gov.uk/ukpga/2003/42/part/1/crosshead...)

It actually is! I presume it implies the morals of the Church of England.

Or the inherent moral view of legislation.

I know. A shocking concept: law has underlying moral values like "it's bad to kill people" and "stealing people's cars is wrong" and so on.

Yeah but murder is written into the law directly and so is theft, not some abstract notion of morality that differs from person to person and culture to culture.

So if paying so sex is made illegal, what happens if you pay your wife for sex? Not sure how this works in other countries, I guess it must be clarified somehow? In the UK it's very common for laws to be vague.