Remember that insurance isn't just to cover the value of the car you buy (in fact, for 3rd party only like this, the value of the car itself doesn't matter a fig!). It also has to recoup the insurance pay outs for thefts and the (potentially limitless) damages to the other cars/buildings/pedestrians that you're going to run in to, and I imagine scooby drivers are just statistically 4 times more likely to have their car nicked or (more likely) plant it into something expensive than TVR drivers are.......
In fact, it's probably the case that cheaper high-performance cars are more likely to be bought by younger, more inexperienced drivers than your more-expensive TVRs, so all other things being equal, I would always expect the insurance cost to be higher.