For interest, when i started this work - back in 1997 - I signed up with some London agencies. This was when I started sex work and these were my very first clients.
Some of those I called wouldn't take me and I found out I was talking wrong! I told them the truth. I had short hair, was short and was 29 years old. I didn't know lying to make myself sound younger and taller was important!
Anyway. I got a little bit of work through 3 agencies. The rate then (not top end) was between £150 to £200 ph. That's almost 30 years ago!
The one agency that was run by a man well he told me he takes the young naive girls for a test run but didn't try it with me.
What I found out about agency work is the kind of guys who book through an agency are looking for a different girl every time. They build up their relationship with the person who runs the agency and with you the working girl.
So yes, their regular clients all want to see you at first, and when they are done, they move on to the next new girl. I never had an awful punters. So, it is worth a try, but don't expect it to stay busy. An excellent central London incall address will help. I regularly did a 2 girl with a girl who lived full-time at her central apartment, and she did OK because she was always there at all times, so the agencies knew they could count on her with each of their new clients.
Some clients - those who don't do this sort of thing often - they don't realise agencies are criminal pimps. So they go to an agency because they think it will be safer and more professional for them.