In reply to climber david:
Aviva and privilidge insured my gf on her own car before she passed, she's 31 though so not sure if that made a difference but I can't see why you would have any trouble. I never had any problems being insured on my own car when I was 17. Once she's passed just phone up and let them know, same as changing your address. Might be worth checking the companies charge for policy amendments isn't extortionate before you sign up as they will charge you for changing her licence type.
It's turned out to be impossible to add her as a named driver to one of my policies, both while she was learning and now she's passed her test which is a PITA. Depending what you drive then if you're renewing yours before she has her test it might be worth factoring that in when you're getting a quote and making sure they'll be happy to add her on at a later date.