I think it works the other way around. If you start your journey outside but the routing takes you through a UK airport you have a maximum 24 hours between arrival and departure to avoid the APD. So, in my case, if I flew from Dublin today and continued to Sydney on Sunday I would be liable but if I fly out within 24 hours (I do) then my journey is deemed to have started outside the UK.

The other thing you need to do is ensure that your trip to the start point isn't part of the same ticket as the longer part or the trip will be seen as starting in the UK which I suppose is fair enough.