When I was 20 I looked at an 1.8 astra coupe with a bertone factory kit. They wouldnt insure me.
What the did insure me on was a 2.6 v6 vectra sri which handled like a boat, had headlights like candles and breaks like feathers.
Insurance is a strange world.
For some insurers, any modification results in an immediate 'decline to quote' - they are simply not interested in quoting. But the same insurer might accept a higher group standard model, despite the overall risk being higher, because it fits their "risk appetite". The problem for many insurers is that they don't have enough understanding of vehicle mods to come up with appropriate rating rules, so they'd rather steer clear altogether. That's left a gap in the market which is generally filled by specialist brokers like Flux and Footman James, to whom insurers delegate the underwriting.