Joe, today both of these journeys incurs the UK APD for the most distant point on the journey, not sector by sector.
It would make sense for the Scots to have jurisdiction on journeys originating in Scotland; GLA-LHR-SIN. But that APD could be less than that for a journey originating at LHR.
On the other hand if they have jurisdiction on journeys where Scotland is the last point of departure in the UK this could have a lower tax from LHR when the journey is indirect than when a direct or non-stop flight is used. (Not very green)
I think we are going to see the rebirth of creative ticketing (hidden city, more distant point, illogical via points). And the airlines probably won't care as its only the government that would suffer.
Why don't these idiots have a chat with ticketing people before they hatch their grandiose schemes.