A tip with elephant:
I'm 19, 2 full years NCB, and am holder of the policy. I only pay £1197 FC with £100 excess on a Mk3 GTi 16valve worth £2500. No mods.
To get it lower, add your parents as named drivers on the policy. Mine shot down by over £400 by just putting my mum and dad on it (even though they never drive it)
Also with Elephant, you earn No Claims as a named driver too, so is worth doing. You must be living in a high risk postcode / high risk job to have a quote that high at 17 mate. I paid £450 TPFT on a 1.3 Fiesta when I was 17 with the NFU!