I ask this question about APD in view of the following statement by Tartin Tom on the Jet2 thread.
The key phrase to note from HMRC is:
Note: If any selection criteria, for seating passengers in the premium cabin or seating, involves extra cost to the passenger for that benefit or the possibility of selection has been offered or advertised prior to the decision to select them, then the operator is deemed to have created more than one class of travel on that aircraft and those passengers are liable for APD at the standard rate.
i.e. if the airline charges extra for the legroom seat then the standard and not the lower rate applies in their eyes.