Pace Ward are who I use. The premium is 1% of the value you give for the car and covers crash damage, fire caused by a crash and accidental fire whilst parked etc.. What it doesn't cover is damage to clutch/engine/gearbox which makes sense really because any old idiot could over-rev and blow something apart. All modifications are covered...
MIS, Reis and Competition Car Insurance are all decent too although I favour Pace Ward.