Sounds like a great idea: you fly, say, SYD-ATH via LHR with a 1h connection, you should really illegally bring your salad, yoghurt, and fruit salad all the way from Australia (you are not allowed to import any of it into the UK, and airside is no exception), let them all stew for the 24ish hours of your long haul trip and enjoy whatever it has become by that time as a 'healthy treat on your LHR-ATH flight. Good thinking!