Originally Posted by
canadiancow
I'm pretty sure the APD doesn't work that way. If you depart the UK, they look at the next stopover/destination, not just the first segment in a connecting itinerary.
UK APD would not be different. UK passenger service charge would be different between a UK to Europe and UK to non-Europe departure. However, flying LHR-FRA-DXB would then attract some German taxes so not really sure there is are any savings.
So I would say the answer to your question of would AAA-BBB-CCC ever be cheaper than AAA-CCC… maybe?