The crew arrive kerbside at the airport over an hour before departure. The delay in getting to the aircraft is due to the TSAs requirements to closely scrutinise the entire crews luggage, shoes, belts etc. Eighteen crew through the checkpoint at once tends to string things out and if you have a particularly officious officer.......

From a pilot perspective getting to the aircraft at 50 mins before departure is plenty of time to do our stuff and is rather more time than we normally get at LHR. Pre-boarding is actually scheduled to start at -35 mins with main boarding shortly after.