Having 1000 pax descend on anyone at short notice isn't the easiest thing to handle at the best of times.
Severe weather warnings, several hours in advance?
Preprepared contingency plans?
Extra personnel called in?
Plans executed, pax disruption reduced?
Nothing to do with T5 on the 31st August, the thunderstorms would have had a similar affect if it was anywhere else.
?????? that is pure speculation.
LHR is runway constrained, whereas some other airports are not. So some other airports may have been able to maintain a hgiher flow rate post event, positively impacting the situation.
A terminal is only as good as the airport's ability to support movements and T5 is a modern terminal at an old (small) site and one must assess the whole package, not just the new shiny bit.