If telling an escort you are a UKP member was a sure guarantee of better service then we'd all be mad NOT to say it. Not sure it's quite as simple as that - I think it all depends on your situation. Certainly if I was married and afraid of being outed as a majority here seem to be then I'd want to be as anonymous as possible and would not bother.
But as a single punter with nothing to lose I have in the past told WGs that I'd come across them via their UKP reviews. Those that are well reviewed here are generally aware of the value of the site although many do have a rather negative view based on the attitudes expressed here by some punters. Hint - if your reviews / comments betray signs of being a mysoginist twat, with limited respect for escorts, probably not smart to let them know who you are on here. It only takes a minute to get a feel for a particular punter and that more than anything I think will guide whether you get a different service or not. I personally can't say whether or not discussing UKP directly resulted in better service for me but I can say I rarely had a bad punt.
The one really bad punt I had where I was
ripped off the girl in question had never heard of UKP. She has now as her subsequent dusty phone resulted in her
reducing her prices from £150ph to £90ph.
As a single guy the worst case scenario for me was that a bad review might result in negative feedback on my AW profile. Ultimately I took the view that if that ever caused a problem I'd just create a new AW ID. I had enough trusted regs that I knew I could quickly build back up positive feedback on a new AW profile if I needed to.
Personally I'd say if you're single and punting and a decent guy then there's nothing to lose by mentioning it in passing, but if you're a dick about it - or just a dick on this site - then it's not going to help.