3.1.4 Transit passengers
Any APD liability applies to the journey as a whole. When an aircraft makes a stop 'en route' and passengers do not change aircraft then no additional duty becomes due for the leg of the journey immediately after the stop.
CLDRVR, this sounds as if it applies to a technical stop once you have commenced the journey rather than using the UK for a technical stop.
The same applies to free upgrades if there is an element of entitlement or priority not enjoyed by other (standard class) passengers, for example in the case of 'perks' enjoyed by airline employees.
This is cruel
g450cpt, you can cut down your liability by measuring your seat pitch and counting the number of passenger seats, including the TOILETS, which I believe are certified passenger seats for takeoff and landing. If your capacity equals 19 passengers and the pitch is less than 40 inches, then you can avoid the highest level of duty.
Mutt