Gutted for you
I wonder if it might be possible to put together a sort of 'passport' package for travelling with airguns.
So what could be included in the 'package' would be - a letter from the manufacturer stating that the gun was manufactured as a sub 12fpe gun, a copy of the letter Rob's got about not being allowed an European firearm cert, perhaps a letter from the BFTA to say that the gun was chrono'd on such and such a date and it was found to be under 12 fpe, and perhaps a letter from the BFTA to say that the shooter is competent and has some level of safety training. Something like that.
Probably still wouldn't help if you've got a complete numpty on check-in but perhaps it might help with a marginal numpty.
Starting to get worried about my trip to NZ in August now