Except transit passengers on one ticket don't have to pay it, but transit passengers on separate tickets do. Again, as I posted upthread, HMRC makes the rules and that's how they've defined it, and so be it, but it is an irritating anomaly when you're on the wrong side of the line. Some countries I believe do levy passenger service charges on transit passengers based on the fact they will require some sort of state infrastructure during their transit (security screening and the like) but APD isn't in that category as it is only levied on ex-UK originating passengers, which but for the "reality" of the way the tickets were issued, I'm not.

Oh well, so be it for the price of using the 2-4-1.