Agree entirely, Chili Monster and IO540. It makes no sense to be underflying a portion of airspace defined by an aerodrome QNH with your altimter set to RPS which could well be susbtantially different. Although perhaps it would be acceptable if flying not above, say, 500 ft of the defining vertical limit of the airspace if on RPS. E.g., if the base starts at 4500' QNH, don't fly above 4000' RPS? There's hardly likely to be around 17 millibar difference between aerodrome QNH and RPS, surely?
Not long ago I flew from Brize to White Waltham. Take-off on aerodrome QFE, change to RPS on leaving the CTR. Advise Benson of my presence, set whatever it was they wanted for MATZ penetration, then the London QNH when leaving the MATZ and finally the White Waltham aerodrome QFE. That's 5 different altimeter settings.....