Ok, have a bit more time now to look up the reference.
It's from MATS part 1, Section 3, Chapter 1 "Approach Control"
1.4.2 Aircraft within an ATZ are required to comply with instructions from the ATC unit. Flight in Class F and G airspace outside the zone is permitted without an ATC clearance. However, controllers may assume that pilots of aircraft flying in the vicinity of the aerodrome in RTF contact with the ATC unit are complying with instructions unless they state otherwise. Controllers are to provide an ATC service accordingly.
Hopefully that remove some of the gray
dp