One of the main reasons insurance companies don�t like to insure non-standard vehicles is that they cannot be certain of repair costs (it may be that a part cannot be sourced within the UK).
Image you have a fully comprehensive policy, if say the windscreen is non-standard, it could cost many times a standard windscreen to have it replaced.
You cannot expect insurance companies to investigate which parts are readily available and which are not � so they either refuse to quote, or quote at a price to cover their possible increased losses.
If the policy has the option of a hire car while your car is being repaired � they could find themselves paying for never ending car hire, while the garage tries to source the parts.