Soliciting and kerb-crawling are illegal in the UK. But being a prostitute is not, and visiting a prostitute is not. Visiting a prostitute is legal even if they’re working in an illegal establishment (and it would seem Mark Oaten’s acquaintance was legal here anyway). And opposing street prostitution while visiting a non-street prostitute is perfectly logically and morally consistent.
Sure, feel free to slate Mark Oaten. But if you do, be aware that you’re condemning him either because you don’t think people who (legally) visit prostitutes should be allowed positions of power, or because you don’t think people who (legally) commit adultery should be allowed positions of power. These are the only two arguments you can use; the one about hypocrisy is bollocks.