>>>Was Manchester effected? was Gatwick effected? Was Stansted Effected
I refer you to my first sentence above:
"Heathrow runs at 98% capacity for landings and take offs, whereas other airports generally operate at below 50%".
To operate to its planned schedules, Heathrow requires to have aircraft movements at a rate of one every 46 seconds at peak times. If the air traffic controllers at NATS (who are entirely independent of Heathrow's staff) are forced (because of greater aircraft separations) to only permit one movement every couple of minutes, it's physically impossible for Heathrow to fit in all of the planned movements.
Airports such as Manchester have several minutes between movements anyway, so they don't suffer from the same congestion problems.