This is the law that covers bikes - don't know if it's the same for cars....

"Silencers - All replacement silencers/exhausts must, for road use, be marked as follows: EU e mark or UNECE E mark e.g. e11 or E11 and an approval number e.g. 007 or BS AU 193/T2 or BS AU 193a:1990/T2 or BS AU 193a 1990/T3 or an international mark that is equivalent to BS or Pre 1985 MC Only. If marked NOT FOR ROAD USE it is not road-legal."

A silencer will only get a CE mark if it's within the legal sound limit, so loud sports pipes don't have the mark.