I imagine it is this; CAP493, S4Ch6.2.2:
Vectoring responsibilities are detailed in Section 1, Chapter 6. Controllers should be aware of the potential for conflictions with aircraft in TRAs adjacent to the boundaries of their airspace, particularly if circumstances have made it necessary to vector an aircraft to be less than 2 miles from the boundary. In such circumstances, consideration should be given to co-ordinating with the appropriate controlling agency. When this cannot be achieved, aircraft should be vectored to be at least 2 miles from the boundary.