Stanstead. The reason no one allowed off is that it would have costs BA to have the aircraft handled and as they have no operations at that airport those charges could be very significant. In addition if you allow one off how do you justify keeping others on board. If large numbers then wish to disembark there are issues around trim and correctly identifying who is still on board.
LHR was closed for so long almost certainly because the fire cover was no longer available – it being all used on a genuine emergency with a real fire. ie the BA762.
Single runway ops would have resumed as soon as the fire cover could be reinstated. However the landing runway and removal of the air frame would take time. The AIB may wish to examine the aircraft in situ and the police may also be treat the scene as a possible crime scene. Moving an aircraft that has deployed the slides is also quite time consuming.
All in all the disruption in terms of closure was probably kept to a minimum however there are clearly questions to be asked about the suitability of using LHR for known events like this. When you at the route actually flown it does suggest that other alternatives were available.
The bottom line is that in such circumstances LHR is always going to struggle and BA in particular will suffer disproportionately to all other carriers due to their dominance at LHR and the nature of their operation.
For me they do not act quickly enough and always seem to over promise and under deliver in disruption. People will understand that they are being delayed and disrupted but they don’t understand when they wait for hours, are boarded onto aircraft and then go nowhere.
I would rather they cancelled and focused on customer service and recovery than try to run short haul flights – only to find at the end of the day that they have no resources to depart flights and no ability to offer any service whatsoever.