If you browse for Belfast -> Heathrow flights on ba.com you are presented with the six BA own-metal flights plus the three Aer Lingus flights under BA codes.
However on aerlingus.com ( once it loads ) you are only shown their three flights...
Joined-up thinking, eh.
City airport press office are still promising nine daily Heathrow flights for Summer 2016. Suggests to me that BA will take-over all Heathrow frequencies and Aer Lingus will shift to leisure routes.