Does the 6 hours exclude enter into Hk Customs and check-in flight to London? If yes, you could take the airport express to the downtown area for a quick view. Otherwise, it is suggested that you stay in the airport where you could shopping and dining, or do some other entertainment. The airport is large so no worry about how to cost time!
Ps. since the H1N1, the security check for international arrival flight may take a little longer time.