Originally Posted by
LHR/MEL/Europe FF
You can check-in for your flight (all day check-in) at the Airport Express terminals in both Central and Kowloon. You need a ticket for the Airport Express to use the service... so you buy one of those, check-in, then are free to do whatever you want and catch the train out to the airport at a convenient time.
Great to know! This is probably exactly what I will do.