They won't publish it, they don't have any negative reviews on their site. Which is the whole point of having a review system...
Happen to me as well, had a bad experience with a girl called Melissa a year ago, sent a bad review to McKenzie same day, it was not posted on their web site but the girl disappeared after few days, probably sacked.
Admiral in London is an example of an agency that does publish negative reviews, but I'm not sure whether there are ever any consequences for the girls' contracts.
External Link/Members Only, for example, has racked up a number of negative reviews, such as
External Link/Members Only, yet they still keep her on the books...
If McKenzies's policy is indeed to sack a girl after one or two really poor reviews then on balance I think that's not necessarily worse than not publishing them, as it arguably puts more pressure on the girls to provide a good service, especially since there are far fewer agencies in Manchester (although in London most girls are linked with several agencies, so that artificially inflates the sense of choice) and in a network such as UKP there's quite a strong collective memory of agency girls, so that a switch in agencies doesn't always mean a fresh start in terms of reputation.