Like many CAA Documents CAP 493 the Manual of Air Traffic Services Part 1 does contain the Regulator's get out clause that controller's can use their own initiative and discretion in response to unusual circumstances but in the real world controllers are generally expected to comply with it whether or not they think it is common sense.
Further south there have been incidents where controllers who "spark" in the way you suggest have found themselves with a problem as the attempt to prioritise leads to pan calls from those moved out of the way. That is why the CAA periodically publishes AICs on the subject.