would need to be measured, not by the portable meter, but by a larger calibrated machine at the nearest police station.
Not entirely true. The portable meter in both the driving and aviation cases is used to provide the Officer with grounds to arrest the individual on suspicion of committing the offence. All portable devices in use are apparently capable of measuring the level with sufficient accuracy to do that for the airline cases as well as the driving ones.
The device at the station is used to provide the evidence to charge the individual. The original guidance from the Authorities on the Transport Act 2003 was that although these station machines were accurate enough to convict in both driving and aviation cases the evidence in aviation cases would initially be from blood or urine samples. It isn't clear whether or not that guidance is still being followed.