Thanks for the replies everyone.
As it happens, there was some confusion about when the planes actually landed, so it turns out both my opponent and I were correct in a sense.
The UK planes actually landed on Greece during Italy’s turn after an adjacent UK carrier was damaged, so the move was legal after all.
My bad for incorrectly believing the planes landed on Greece during the UK turn, and neither of us realized what the other was talking about. :-D
Good to see our fellow AAAers always ready to help out with good answers, though, so thanks again all. :-)