He won the world road race championship (making him world champion). The last time a Briton managed this was in 1965. Additionally, he won the green jersey at the Tour de France which has never been done by a Briton. Ever.
Basically his achievements are unprecedented in British cycling. british cycling is currently in the best shape it has ever been, Bradley Wiggins, Sir Chris Hoy, Victoria Pendleton et al... Cavendish is the jewel in that crown.