The overflight Rule for congested areas is now embodied in Single European Rules of the Air (SERA) Regulation (EU) 923/2012.
SERA.3105 Minimum heights Except when necessary for take-off or landing, or except by permission from the competent authority, aircraft shall not be flown over the congested areas of cities, towns or settlements or over an open-air assembly of persons, unless at such a height as will permit, in the event of an emergency arising, a landing to be made without undue hazard to persons or property on the surface......................
This superseded the previous UK ANO provisions.
Reference to this whole issue, at least, on my part, is tongue in cheek. Cannot imagine for one moment that the CAA would require it. If they did they would have originated the NOTAM citing the SERA Rule. Consequently IMHO EGNC management, in the absence of an operational ATC unit and ATZ are acting beyond their legal rights to mandate a minimum overflight level.