I also think that most (if not all) road noise will be down to the tyres, although i must be completely honest and say that British roads are not the quietest on the planet.
As an British ex-pat living in Holland i do a lot of driving on the continent and often come to England and Wales for family visits and the road surface in Britain is, in short, abominable.
The M25, for example, was obviously put down by a number of contracters, all with their own vision on how a motorway should be built.
Basically, even if you fitted cushions to your car, your roads will produce awfull amounts of noise. It's always a relief when i cross the border between Belgium and Holland and everything just goes quiet (Belgium is second or third worst surface).
So, sorry, the only way to get rid of the noise is to emigrate to the Netherlands
