I think you'll find that based mainly on the statistics of WRX Sti's crashed vs WR1's crashed, you'll probably find that the WRX owner is considerably more likely to be involved in an accident than the WR1 owner.
Insurance companies are really just statistic-based profit-making machines - how that calculates out on an individual basis may seem crazy, but it's all based around the statistical algorithm that they use to maximise their profits.
If you're the type of person with £18k ready to spend on a 2004 WR1, then you're probably statistically less likely to stack into something expensive (like an office building, eh? ) than the buyer of a £7 STi......