They knew that anyway, when I used to work in central London at universities most of the buildings were wired in a ring from UK power networks, we occasionally had power cuts and faults but due to the ring just one part could be isolated and normal service renewed.
And all our critical stuff was covered by generators and UPS equipment, I would be shocked if Heathrow didn't have generators for critical kit.
But one thing is for sure, there will soon be backups in place very soon. I fell sorry for the people working there, must of been mayhem.