There's no such thing as 'Regional QNH' any more.
I think the plan is to get rid of Regional Pressure Settings when the TA is raised. Aircraft inside controlled airspace will be given a QNH; aircraft outside should fly on the nearest reported QNH as they do nowadays when flying below a TMA.
I don't know what the RAF will do. I think their present procedures are to remain on QFE up to 3,000 ft then change to 1013. If remaining below 3,000 ft they use RPS when outside the MATZ.