I can’t imagine the RUK fleet being massive. They are only needed to fly domestic routes (of which there aren’t any at the moment) and routes to non EASA countries. Anything between U.K. and EASA can be flown on the Irish register. I imagine with the exception of STN, those routes could be covered by 1 aircraft at most bases.
i believe the SOPs are almost identical. A few minor things as mandated by the relevant NAA but day to day I believe is the same. ‘OCC’ from RYR to Malta is a 15 minute e-learning course if I remember correctly. Can’t speak to Buzz or RUK.