I do RBs often, and generally get a very good response if I simply say as part of the booking request that a face photo is helpful to me when making a decision. If you put it a bit like that I think you're unlikely to get anyone slagging you off (not everyone will send a photo, but if that's the case you can always ignore them).
The way you describe your RB says that all you are willing to guarantee is a fee of £50. I can see why that might not get a brilliant reply, especially if the girl has to travel at all to get to the booking. It may be a good rate for 15 minutes, but it's not a massive amount of money in itself, especially once you add in the time the girl will take to get ready and get to the bar where you want to meet.
Personally, I think it's fair enough to discuss shortening a booking for a reduced fee only if the WG is the one suggesting cutting it short, or if agreed services were not provided. But what someone looks like and whether you find them attractive is not a service, it's a risk which you choose to take when you punt (especially using an RB).
Do what you can beforehand to make sure it will be OK (photo, exchange email messages and arrange to speak before you meet). There are no guarantees but if you put a bit of effort into it you are much less likely to make expensive mistakes.
