The answer lies on the availability of Aerodrome QNH. In the case of a TMA, or a CTA where the base is defined as an altitude, there will always be an associated aerodrome or aerodromes. You should use the aerodrome QNH to ensure that you do not infringe the CTA.
With an Airway, the situation is different. The Airway could be a considerable distance from an aerodrome reporting QNH. Thus aerodrome QNH may not always be available. Consequently, one can use the Regional PressureSetting (RPS) but it is recomended that an aerodrome QNH is used if available.
Airways will have a base defined as an altitude in two circumstances;
1. Simply a base altitude e.g. 3000 - FL245. Easy one there.
or
2. Complicated - a base defined as a Flight level but an assurance that the base will never be lower than a certain altitude. e.g. FL65 - FL245 minimum ALT 5500.
ATC will use the RPS to determine the minimum level available on the airway (500ft above the base) and Sectors where this happens will have a chart of RPS value against minimum level available.
Simple answer - get rid of RPS unless you can find an airway in the UK defined as a base altitude and more than 50nm from where one can obtain an aerodrome QNH. It can't be that safe when even the AIP can't be relied on for an exact answer.
Regards,
DFC