There is one other aspect to why there are differences between the "stated" angle and the "measured" angle
darkroomsource,
I doubt that there is a significant error due to map projection. I'm not sure about UK maps, but the aeronautical VFR and IFR charts, that I am aware of, use the Lambert Conformal Conic projection. This has a useful property that straight lines on the map approximate great circles, unlike plotting charts and nautical charts, which use the Mercator projection. On Mercator charts, a straight line is a rhumb line.