UKAIP ENR 1-7-2
5.2.2
In flight or below 3000' amsl pilots may use any desired setting. However pilots flying beneath a TMA or CTA should use the QNH of an aerodrome situated beneath that area when flying beneath the transition zone.....Pilots in flight at or below 3000' on an advisory route should set altimeters to the appropriate Regional Pressure Setting.
This seems to indicate that within a TMA or CTA the aerodrome QNH must be used but outside of this regional pressure should be used.
It may be safer or even more sensible to fly at a nearby airfield QNH even if you are not within their TMA or CTA but I am looking for clarification of the true position - If I was flying at 2,900' on the Regional Pressure Setting and 3,200' on the local aerodrome QNH would I oficially be busting the airway if it was marked on the chart +3000' ?