Even later in the day I have had flights (often lightly loaded and boarded mainly by people who know how to travel) depart 5-10 minutes early, but the gate closed on time - 20 minutes before departure. It doesn't just happen first thing. Sometimes, mirabile dictu, aircraft are ready to depart without delay later in the day at Heathrow.

It simply took less than 20 minutes after gate close to get everyone down the airbridge and onto the aircraft, seated with all their stuff stowed, paperwork passed through the door, pushback crew [1] all ready, and off we went.

That doesn't mean the departure time was moved earlier. It also means that anyone showing up 19 minutes before departure was going to make the same walk of sorrow and disappointment back to a service desk or telephone to ask for rebooking.

[1] Army of One, with Mototok