Insurance is all about risk and that comes from statistics.
Big sweeping generalisation here..
Statistically speaking:
A 17 year old will have more accidents.
A 17 year old is likely to drive a small engine.
People who recently passed their test have more accidents.
A male is more likely to have accidents.
The more miles you do increase the chance of an accident.
So you got to think in those terms, insurance is not strictly directly proportional to the value of the car or engine size. It is also a risk game for them to insure you.
That is of course the post code, area you live in with its crime rate also affects it, as well as where you park it. It's not just the possibility of an accident, also the possibility of you making a claim - theft, fire, criminal damage etc.
So do something a little different, get something a teenager stay away from, think outside the box.
p.s. £600 is cheap if you have no named drivers on it.