Up in the midlands all our Southbound departures overflying london were also delayed.
The reason given to us was that the Swannick ATC controllers had to empty the Heathrow stacks and divert all the aircraft away which made the airspace very busy as lots of different aircraft in all the different stacks all look to go in different directions and they have to juggle them around each other.
Essentially "nobody else can join in until we've finished sorting out the players already in the area"