Questions will doubtless be asked
"Questions will doubtless be asked about why Heathrow doesn’t possess sufficient generation capacity to cover and event of this sort – like datacenters do."
I'm sure safety-critical airport functions (eg air traffic control, radar, runway lights and radio navigation aids) do have backup power so they can continue for a while. Buit all the things needed to support "customers" (which does not include perople arriving in an emergency) is probably approaching the power requirements of a small town.
I think the more interesting question is why the substation is apparently a single point of failure. I guess the answer is a combination of increaed power consumption and minimal infrastructure investment to support same, perhaps in part due to those datacentres.