Insurance companies have some very strange policies, I bought a second car last year (98mk2 mx-5) completely standard admiral £761 0-NCB(using my NCB on the zed), just re-insured it with sky/chaucer, £456 declared mods: supercharger +70-80bhp, rollbar, wheels, BBK, suspension, exhaust it makes no-sense at all.
Just keep searching mate, perhaps mildly adjust your anticipated milage a little