In reply to mike123:
I've just been looking into this, as daughter will be 17 in a month. One important point is that provisional licence is whole different wold from full licence. With some shopping around I've got a quote to add my daughter onto my policy for about £60 more than my normal renewal price. When she passes her test, it's £6000. She won't be driving my car once she has a licence
So for now I'm adding her on to my policy for some practice between lessons, and she can start thinking about how much she really wants a car once she passes her test.
One thing to watch out for when your insurance comes up for renewal: Lots of companies, including my current insurer, won't insure anyone under 21 on my car. So if I'd renewed as normal I wouldn't have been able to add her when she turns 17. I've checked and made sure my new insurer will take teenagers.