Due when the aircraft first takes off on the passenger’s flight and is payable by the aircraft operator.
Is this a change? I believe APD was previously payable by the passenger, based on the most distant point on their ticket (which encouraged split ticketing).
This alone could have a lot of ramifications, mostly negative to UK airlines. Do they know what they are doing? (No need to answer that - I know they don't)
Edited to add. This isn't workable ... What charge per passenger would QF/BA pay for a flight destined to Australia via SIN (or BKK)? It would seem that the either the Australia APD would never be paid, or that the airline would be charged the higher amount for all passengers, including those destined to SIN.