APD arrived because the Brits, like most nations, do not like Income Tax. They have made it clear across 40+ years that anyone promising to increase taxes will not get elected. So Indirect Taxation has become the game. Brits now pay a tax on insuring their car as well as insuring their car, etcetera.
At LHR they changed their pricing system from a 'size of aircraft' to 'how many seats on an aircraft'. Everyone tries to slice the cucumber into thinner slices so as to make it 'dissappear'. Sometimes, a passenger collectes several slices on one journey - sometimes they don't.
Do not expect indirect taxation to change any time soon. Unless you vote to put up direct taxes and VAT.