Be ready to leave without thinking. If there is ANYTHING you don't like about what you see, just turn back and walk away. This way all you really risk if your time.
I used to live in Leicester and used vivastreet. I'd go to a place based on photos, look at the woman and in 80% cases just tell her "you are not the woman in the photos, goodbye". Most SPs didn't like it, so what? I would just drive to the next one until I found somebody I liked. Over a year of time I met all the SPs in the area and I could recognize their profiles even if they changed photos, they are usually too lazy to change descriptions. You just get an instinct of what looks too good to be true. (Hint: every SP has a problem, otherwise they would not be SPs. A genuinely pretty, smart girl can just go on a walk in Chelsea and marry a guy who makes 200k a year.)
Never had any problem at all. I found some real gems, like very pretty and fun 20year olds charging 70ph.
Now, London is different as driving around takes forever here. But the first rule still applies. Look at her, look at her body language and make a deliberate decision before you reach for your wallet: do you want to fuck this woman and does she make a good impression?