This whole issue stems from the idiotic organisation of airspace and traffic services in the UK. At all other places I know, each one given point, as defined by latitude/longitude/elevation, belongs to one and exactly one part of airspace, with one and exactly one service in charge. So, depending on where you are, you call the relevant service and they tell you what QNH to set. Solved. Why the UK absolutely wants all this vagueness and confusion is beyond me.