I thought Aer Lingus had got a UK AOC for their Manchester long haul operation. Would they not be able to use this for their Belfast flights if they wanted to?
I guess they would then have to put one (or more) A320s onto that UK AOC, an issue in its own right and one which reduces fleet flexibility.
I do wonder though how it is only an issue now, and not before from the day of the end of EU membership.