Rutan16, what you've written is just complete rubbish and shows a total mis-understanding of the law.
The liability to calculate and remit APD to HMRC rests upon the airline. The airline is obliged to remit the correct APD for the ticket it has sold to the customer and in the case of a passenger on a DIY connection flying EDI-DUB-EDI, it correctly remit short-haul APD even if that customer has a separate ticket DUB-BDL-DUB. If the liability rested with the individual passenger then there would be an argument that your statements would be correct. However, it does not. The airline carrying the passenger on departure from the UK is correctly calculating its liability to pay short-haul APD for the EDI-DUB leg which is what it has sold to that customer in that transaction. Only where a single or conjunction ticket is sold for the EDI-DUB-BDL-DUB-EDI roundtrip does the higher APD become payable.
If one takes your argument to its logical conclusion, then a self-connect passenger flying AMS-LGW-OAK-LGW-AMS on EZY and DY would be able to reclaim (or just not pay) the UK APD paid on the easyJet ticket AMS-LGW-AMS and the Norwegian ticket LGW-OAK-LGW - so a not insubstantial sum. They cannot. Spreadsheet Phil has the lot and keeps it.
I don't mind folk getting on their high horses, honestly, but it does stretch the boundaries when the factual basis of such postings is so far off the mark!