Let me re-phrase that for you:
It only took down the airport because the airport clearly did insufficient capacity planning in terms of backup mechanisms.
How can I be so confident ?
Because that exact same substation serves a number of large datacentres in the vicinity.
Due to the grid constraints previously discussed here, many of those same datacentres take ALL their feeds (A,B,C etc.) off that one substation, the only difference is the cables are diversely routed. Not their choice, it was imposed on them by the grid.
They have ALL been without ANY electrical feeds all day. I know that for a fact.
HOWEVER, those same datacentres have been running non-stop like nothing happened. I know that for a fact.
Why, because they have N+1 generators (which are regularly tested) with at least 48 hours of fuel, which was topped up this morning as soon as it became clear it was a major incident and with multiple fuel deliveries already pre-scheduled from multiple independent suppliers. I know that for a fact.
The grid are of course very busy trying to work some magic to re-arrange things to get the datacentres back online. Meanwhile the datacentres are very happy to keep ticking away on generator power for as long as it takes, its not a problem for them, its an event they plan, prepare and practice for.
Heathrow could have done the same. They could have added generator plants here and there over the years when they re-built terminals and such like.
They didn't, or at least they didn't do so with sufficient capacity.
Maybe Heathrow also fell behind on their generator maintenance and testing regimes. Who knows...
There are people out there who say it is because their motto is "spend little, charge a lot", so they did de-minimis, prefering to focus on maximising revenue generating space. I could not possibly comment.