Ryanair did have a UK AOC when it was operating EA Airlines or some name like that from Luton in late 80's early 90's.
If there is an issue I see Ryanair getting a UK AOC relatively easily.
If UK Govt plays hardball then could see BA being a loser as reckon a few countrys would like to ban their use of Heathrow as a transit airport.
If BA couldn't sell through tickets then they would have to charge APD to everyone.