I had a LHR - DUB flight which BA cancelled - there were a couple of BA options, and several Aer Lingus options between the cancelled flight and the one I ended up getting on - 3 1/4 hours later. They said the cancellation was due to air traffic control issues so wasn't eligible, but gave me 30,000 Avios (I think I'd paid 4,500 Avios for my flight... :-)