The United Kingdom's Air Passenger Duty ("departure tax") is quite straightforward.
For economy class travel, the duty is £10 short-haul (domestic and European) and £40 long-haul (everywhere else). For travel in premium classes, the rate is doubled, i.e. £20 short-haul and £80 long-haul.
It's a tax, pure and simple - a revenue-raising exercise. There is no need to justify it or explain what the money is spent on.
Any charges over and above the APD are applied by the airline itself (e.g. security surcharge, fuel surcharge or Ryanair's wheelchair surcharge) and/or the airport operator.