With indies, it's not really enforced even in Sweden anymore, there have been very few prosecutions IIRC.
If Harridan has her way and this becomes law in England in a few years time, I will largely ignore it (despite being a married man with kids). I'll be a bit more careful, and wary of sting operations (new escorts with zero feedback will have a tough time).
As for stakeouts, I imagine the conversation would go something like this:
"Yes, officer, I was visiting this woman. She's a former prostitute, and I was once a client of hers, back when it was legal. We've become good friends, we like to get together for a drink and a chat, and sometimes we still have a good shag. Oh, she's still on the game? Funny, she didn't mention that".
Very unlikely they could film the money changing hands, so either I'm skint when they catch me on the way out, or I have a fair bit of cash when they catch me on the way in. I'll stop putting it in a separate envelope and pay her out of my wallet. What is an illegal amount of cash to carry in a wallet?
The only real chance of gathering evidence would be through electronic eavesdropping. If you're visiting her at the same time as "randyshagger" (or whatever) was scheduled to do so, that might be difficult to explain. But as being a prostitute won't be illegal, this would involve spying on an innocent person. This would need anti-terrorist powers that the police don't usually have, Harridan would have to argue that prostitution = trafficking = organised crime = terrorism. A bit of a stretch, to put it mildly.