And how much shorter is 'Chatham QNH' than 'Chatham RPS' (or Chatham Regional)?
Let's face it, practical RTF has little to do with the esoterics of QNH versus RPS. Those who use the radio are all human (even the air trafficers) and will use whatever phraseology suits them, no matter what appears in CAP413, ICAO Annexes or any other publication that nobody ever reads.
The original question was "Is there such a thing as Regional QNH?". The pedantically correct answer is 'No, there isn't'. But who the hell cares?
In the current climate, don't we all have far more important things to think about?