Hey there, no I am not in the UK and am actually in the US. The agency I work with allows the escort to negotiate, which is great, however it can prove frustrating due to walking into a booking with some unease. I almost feel I must just say the total amount (my fee plus the agency fee) when collecting up front, because you are more than correct. Negotiating while in a state of undress is not in your favor.
I'm not surprised - this isn't 'great' in any way, it's just the agency's way of making you do some of the hard work that you're employing and
paying them to do for you.
They know they're breaking the law (just as they would be here or in most other places) and they're trying to make sure they can pass off some of the trouble onto you should the law ever take an interest in them - the likelihood of this happening depends on lots of things including how they operate and where specifically you are. Is there a reason you don't want to work independently? We don't recommend agency work here anyway, but in the US you've the additional problem of being on the wrong side of the law yourself and it's pretty clear that your agency won't mind taking you down with them.
The key is never to say anything either on the phone or in person that could be construed as 'I will have sex with you for money'. If the agency are doing proper screening, then in theory it shouldn't matter, but most won't. If you want to walk in and say it's $XXX for the hour that shouldn't be a problem, but if a punter starts asking you to list services, be wary. Again, recommending suitable ladies and putting up what they call things like 'Enjoys' lists is their job, not yours.
The good news is that decent clients know the score and will be wary of stings themselves, but it's far from a situation most of us would relish, certainly.