It's a combination of various mods....anything over a 10% bhp increase in going to attract a massive loading. The value of a bodykit in relation to the value of the vehicle, same story for alloys. Boost controllers are just a big no no.
Random example.....If you've got a car that you're declaring a value of £5k for.....if the wheels are worth £2k and the bodykit is worth £1k then, in effect, the mods are worth more than the car itself.....underwriters don't like that. Add in some power gains and then you're just adding loadings on top of loadings and you end up with silly prices.
In the main, if you've got a 'fairly' normal car and have just done the basics like exhaust, filters etc then we're going to be very competitive. Start adding power gains of more than 10% and the price starts going up...lots...Hope this helps :)