Some might say that all (well most) insurance is a rip-off/scam
There will be a number of factors an insurance company will consider when providing insurance (though probably not limited to the following!):
Personal details (from where you live to if you smoke/are single/how old you are)
Car particulars (Model, Security, Cost/Availability of parts, Colour)
Extras (Anything non-standard and it's associated costs/perceived change to driving style)
Statistics (Based on the above information, how often are similar cars involved in accidents/claims?)
Of course, some insurance companies can't be bothered with "extras" (hassle or perceived liability of the relevant drivers) so either won't insure or charge some nominal extra (e.g. the £2k above) to force drivers to pay up or go elsewhere.
It would be interesting (maybe under the freedom of information act?!?) to see the statistics regarding accidents and maybe how modifications affect them... Despite that, there will always be a bit of a black-art involved with the numbers they come up with!