Anyone flying below a portion of airspace defined by aerodrome QNH cannot use anything other than that setting to guarantee safe clearance.
The problem stems often from the military's atrocious understanding of sub-division of UK airspace. For example, many pilots from Benson blindly setting 1013 on passing 3000 ft QFE with absolutely no regard for the base of the London TMA overhead being based on the London QNH. 5 miles south-east of the aerodrome at 3203 ft amsl, how many of them knew that the LTMA was less than 300 ft above them? Answer - none. Because they haven't a clue and aren't taught about such things....
I tried to get the TA at Benson changed to the same as for the LTMA but was told "It wasn't standard procedure"......