AH - Exactly!
It may not be in law yet but from 01 Jan there will either be no agreement (nothing flies) an agreement or an emergency temporary agreement. The CAA make it clear in their podcasts (and no doubt in conversations with airlines) that the Air Service Agreement between the UK and EU will only be third and fourth freedoms ("97%Written up, just awaiting political sign off"). Airlines wishing to operate under this agreement will need to have a UK AOC. I am not a Brexit fan but this is the point of taking back control, leaving EASA and bolstering our own regulator. It needs something to regulate and the rest of the UK airlines would not be happy if there wasn't a level playing field. Apart form the cost, Ryanair are probably just miffed because they will no longer be able to pool their airframes across Europe / UK. The UK airframes on the G-Reg will have to be ring fenced for UK based ops only. They set up Ryanair UK so they must have been aware of what was coming and would have had a lot of conversations with the CAA as they did this.