Am still curious what went into that decision.
My best guess...
1. Early hours of the morning in London. Out of the blue LHR closed..completely, with
no estimate as to when it might open again.
2. Multiple flights from multiple airlines already inbound to LHR from the likes of the Far East, the States/Africa/India planned to arrive in a few hours, just after the night curfew ends.
3. If LHR cannot open for arrivals after the night curfew then there will be mass diversions of those inbound flights with the preferred options close to London (Gatwick, Stansted, Luton, Cardiff etc ....getting full very quickly usually down to lack of parking spaces. Later arrivals are therefore at risk of diverting to far flung exotic alternates ....and then, if and when LHR opens, you’ve got dozens of aircraft out of position and crews potentially out of hours and unable to recover aircraft and passengers to London.
Given the above if you were in BA company operations at LHR when it all went wrong would you sit back and continue to allow those flights on the ground at outstations such as IAD to depart towards London on schedule and hope that LHR opens up in time for their arrival?.....Or would you ground hold those flights until you have some idea of if/when normal service of sorts will resume at LHR?