Mutt, you are referring to pax originating from the UK, then you are right, the transit/tech stop is not the one that counts for ADP, the final destination is.
However you are originating from the Gulf, tech stop in the UK and then go on, that is considered a case B, and there is no APD for your pax.
The same applies to for example US carriers that use LHR as a hub, if the UK would charge APD on those pax, that would put LHR out of business pretty quickly as most if not all would switch to AMS. They put that loophole in explicitly to protect the hub status of LHR and I don't see that changing, ever.