I've been to two so far that have asked small deposits. Both were legit and in both cases we spoke on the phone. One was a girl who was quite busy and wanted to make sure that she was only taking bookings from people that were going to turn up. Paid by Paypal but put for goods and services so I could chargeback if needed. But the meet went ahead. The other one was a massage asking deposits just for one particular day because her and her partner needed to be away as early as possible, didn't mind waiting if people were going to turn up and asked a £5 deposit just to ensure they weren't waiting for nothing
On the other hand, I've had four that have asked for £30 deposits, won't talk on the phone and when asked for details to try and clarify they are where they say they are, they're unable to do so. You're right to be wary. Caveat Emptor