Barkingmad has hit the nail on the head; the UK CAA has a responsibility for oversight of ALL CAT operations in the UK regardless of the State of registry.
However, as we all now know, the UK CAA has become a rudderless, ineffectual organisation of time-serving individuals who, apart from a small and reducing number of exceptions, are unemployable elsewhere due to a fatal combination of lack of experience and lack of knowledge.
I know, a predictable response, but derived from very close contact with most of the SRG, especially over the last 5 years. I know many who have quit in disgust, especially in the Airworthiness side of things but also in Aerodromes, and I know just how bad are those who oozed their way into the shoes they cannot fill.
So don't look to the UK CAA for any action over Ryanair's operations in the UK, even if it were necessary. Last time I was involved, Ryanair's maintenance standards were extremely high, for very sound financial reasons at least, whether or not their Flight Ops management is questionable.