Hesitate to get involved in the regional pressure setting dog's dinner but the plain fact is that the airspace beneath TMAs and CTAs, except airways and Daventry and Worthing CTAs, does not form part of the Altimeter Setting Region system and pilots should use the QNH of an adjacent aerodrome. Therefore in my opinion the person who would be responsible for an unauthorised excursion into CAS by an aircraft flying on the RPS is the pilot.
In fact back in the late 80s early 90s there were a couple of Airproxes caused by military aircraft aborting low-level missions and climbing into controlled airspace whilst using the RPS because they had no means of obtaining a local QNH. As far as I know this particular issue has never been satisfactorily addressed despite CAA Recommendations to the MOD at the time, although I stand to be corrected.