As far as I'm aware, kerb crawling is illegal and can lead to your vehicle being confiscated. A girl soliciting 'trade' on the street is also acting illegally. If you are busted in a brothel raid it's not illegal that you are there paying for sex, although if the girl is being coerced in any way you will be in a heap of s**t. If you refuse to give your name and address you will be arrested, and even if you do give it there is the possibility you will be taken to the nick while your id is checked.
But paying for sex, with an unforced and over 18 girl is not illegal in mainland UK ...............yet.