A small deposit for a mistress is not unusual, you pay your money and take your chances and they deter timewasters. However, most of these SPs have a reputation to maintain (there needs to be trust in a sub-dom session), and they are not going to risk it for your £50 or whatever.
More worryingly, I would say £300/hr is a lot of money for a not-sure punt of any type. She would have to be very good for that sort of rate, and if you have no evidence of this, then there are cheaper options.