Give us some details: What's done to the car, why did they knock you back, what reason did they cite etc.
I insured an engineered CA18 Turbo Bluebird wagon with Vigil car insurance when I was about 20-21. Fire and Theft, about 800 bucks a year.
Just Cars have huge excesses, and their policies are quite inflexible. I insured my '78 911 with them because Shannons weren't convinced (when I was under 25) that I was a genuine Porsche fan, instead of a cashed up young bloke.
Shannons are looking at hardcore enthusiasts, not blokes that have tricked up their old jap banger. They will ask you some difficult questions before they insure you. As far as they're concerned, you are driving a common as muck corolla with a big engine, definitely not the market they're after.
CGU insured my Ke16 full comp for about 250 bucks. When I put the 911 onto Shannons after 1 year of Just Cars, I put the rolla on with them for about the same money, 250 bucks, covered for 5000!