Ok, here goes... there are 2 stereo fixings to fit the UK spec IS - the S radio has an inbuilt amplifier and will not fit the SE or Sport, which have an amplifier built into the car - I know 'cos I've tried...
Next thing - some RS have the amp and some don't.. not sure on the rhyme and reason on this.
The way to tell is to open the glovebox and peer into the front wing - the amp is a silver box mounted towards the door end of the wing.
The good news is that if the amp is there, the Se radio will go straight in without changing the fixings, but when they cost 650 quid new, it might be worth looking around to see what else is available.