Marina
I recommend Actors' Yearbook as a start. It's a bit less daunting than Contacts. It does list a lot of agents and it also has helpful hints on how and when to approach them and lists individual agents' preferences for how they like submissions.
The difficulty your dd faces - and I'm sure she's well aware of it - is that she is in competition with hundreds and hundreds of drama school graduates who have had the advantages of showcases and final year productions to try to interest agents (as well as the training, of course, which for some agents will be of very considerable interest/importance).
All the good adult agencies will want to see her in something before taking her on. Occasionally new agents do audition adults - but most established agents take people on after seeing them on stage or on screen or from watching a showreel, or sometimes after recommendation from someone they trust in the industry.
She can still approach agents though - if her CV and photo are sufficiently interesting they may invite her in for a chat. They might take her on on a trial basis, or they might say "let us know when you are in something we can come and see".
If she is not going on to further training then she could concentrate on getting a showreel together. That can be a useful first step on the road to representation. Agents will almost always want their clients to be in Spotlight - and that is another hurdle for actors who have not trained on an accredited course. It is possible that Spotlight will consider her professional credits as a child performer as sufficient evidence of professional work but you would probably have check that out with them.
https://www.spotlight.com/join/eligibility.asp?book=2
It is also possible to get work without an agent. Spotlight entry will make that a little easier too - though she could start by looking for jobs on some of the other casting sites like CCP and Starnow and Mandy etc.
