Apparently there is a serious problem with UK Air Traffic Control computers. The BBC are reporting the problem here.
Edinburgh, Manchester, Birmingham, Southampton, Luton and London City airports have also reported delays.
Current delays at Heathrow appear to be about 2 hours:
Luton appears to be as bad:
And London City seems to have cancelled a lot of flights:
Check with your airline if you are travelling or be ready to do Christmas Shopping the terminal.