Spoke to mine about this yesterday as it happens, as wasn't sure whether I'm supposed to declare all the options that we've gone for on our 1.4 Sport, like the leather and glass roof and larger alloys etc.
I was told by my insurance company that they know what optional extras are added on/included in a brand new car, through the DVLA apparently by checking against the registration plate. Basically, she told me that I didn't need to inform them because they already have that info just by going by the number plate. I hope I make sense.
She said that the only time you'd need to inform the DVLA is for eg, if you bought a 2nd hand car, or a new car and then added something on 'after' you have taken ownership of it, ie new car bought in April 2008, then added bigger alloys in June 2008.