Quite simply, this is because I had reason to be in Copenhagen for some time prior to my TATL trip. That's hardly a roundabout itinerary. It's called: "starting out from where you are". If BA and the UK APD people want to make it cheaper: well, thanks a lot. Rather than abuse the ex-EU advantages by omitting the LHR-CPH portion, I'm going back to CPH (where I choose to get some specialty purchases at Kastrup). What would you have me do? Give up my interlining at LHR? Pay unnecessary APD? Cheat on the ticket conditions? Buy a single ticket from LHR-BCN via CPH (with whom)?