Doesn't any of the Air Passenger Duty find its way into funding government-required services like.....Immigration?
Between that and the PSC you would have thought so, but knowing the accountants it doesn't work that way, especially as both are payable on the outbound leg. If more people are getting routings that mean outbound to London but then train or loco to CDG or AMS for a return home that avoids LH APD, maybe there is a short fall?