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/