I'm with Adrian Flux too, no mods to declare, but driving an R56 factory
JCW... definitely the cheapest of the bunch, as they have been on my last couple of cars. I'm confident however, from speaking to other owners, that they would be willing to cover a reasonable number of performance modifications with little or no change in premium (other than the usual admin fee perhaps, which you can usually blag yourself out of, especially if you've been a customer for a while).
My old R53 Works was insured by a company who would cover a sensible list of performance or cosmetic modifications for little or no premium (they did the same for several friends too), however owing to problems I encountered with them when changing vehicles, I'm reluctant to make a recommendation. Will be interesting to see if anyone else brings them up.
For what it's worth, Liverpool Victoria (unless they've changed their rules) will cover any non-performance mods free of charge... wheels, bodykit, graphics, etc.