I've paid a deposit before and it turned out fine. I just decided I was comfortable risking it because I got good vibes and also she had made a shit load of amateur porn and the reason for the deposit was understandable as she lived in the middle of nowhere (and it was only for in calls). I was 99% sure beforehand she was legit though, but I still asked about deposits on UKP first out of diligence. Vorian said she gave good service so my hard on at that point said fuck it. Turned out to be the best punt I've ever had (so far).
External Link/Members OnlyI've given a different girl some money via bank transfer (£30 or so) for petrol for an out call but she was going to get £200 in total so I figured she'd show (and she did).
But like I say, all at my risk, but that's how I roll. In general it's a terrible idea, the girls are earning plenty already and the risks they take (that a deposit could cover them for - i.e. TWs) are no worse than those we punters take. For some who don't want address collectors stalking them or whose husbands insist on it or who have families they need to protect it makes sense though.
A deposit is nice in a way as it's some comfort that they'll honour the booking and won't drop you for someone else, but I do wonder what would happen if they were ill on the day and had to reschedule. I might well consider a £5 or something in advance for a profile that doesn't have a PG or checks out well. I've been on wild goose chases for cheap punts before where I was running late and wished I'd paid a deposit so I knew for sure they'd honour my booking before.
OP the profile you linked to doesn't strike me personally as worth the hassle or risk but after looking at all the other girls in Greater London if you still have a hard on just for her, phone her up and see how you click over the phone before handing over cash in advance.