Great post BP, I'm sure the OP appreciates someone taking the time to write something so extensive.
Just one little point of clarification the UK itself does not have exit controls so it is not necessary to depart from a customs airfield or indeed file a GAR for the outbound flight expect if departing to areas designated under the Terrorism Act 2000 like Northern Ireland and the Republic, the Channel Islands and Isle of Man.
Most other airports in Europe yes if leaving the Schengen area (to go back to the UK for example) must be from a customs airport.