You couldn't routinely run an EU registered airliner in a UK AOC, but the UK/EU agreement sections on aviation (a) allow for short term expediency, and (b) explicitly exclude light GA activities from the rules. So basically there's no absolute prohibition, and it's going to depend upon how the CAA feel on the matter. I don't believe that there's any rule either permitting, or prohibiting this.
Ultimately however, it'll be about the same rules as would apply to use of an N-reg aeroplane. I believe that's acceptable with written permission from both FAA and CAA, and has been done.
G