The FAA aren't allowed to write ROYAL NAVY on their GR9s as they're said to be RAF assets.....
Incorrect.
The RAF aren't allowed to paint ROYAL AIR FORCE on the side of 'theirs' either.
To be pedantic, I believe the historical convention regarding aircraft markings is that only RAF aircraft carry fin flashes; FAA and AAC aircraft carry "ROYAL NAVY" or "ARMY" titles instead.
AFAIK, all JFH aircraft have fin flashes.