I'm pretty sure Her Majesty's Customs & Excise (or what ever they call themselves these days) ignores the fact that there are two separate tickets and the only consideration is how long any intermediate connection is. If it less than the time stipulated in the rules then it is considered a transit and the APD through to the final destination is to be collected.
And BTW "Connection ticket or Conjunction tickets" only come into play if your are transiting through the UK are are seeking to avoid paying any APD
Edited to add: I have tracked the relevant web page, and now I am not so sure. See
here
In section 2.1.1
APD shall be charged on the carriage of each chargeable passenger at the rate appropriate for the place where the passengers’ journey ends.
Journey means the journey from the passenger’s original place of departure to the final place of destination.
The original place of departure and the final place of destination mean the original place of departure and the final place of destination indicated on the ticket.
But:
- where a journey includes two or more flights, and
- any of those flights is not followed by a connected flight,
the journey ends where the first flight not followed by a connected flight ends. See section 4.