GoneWest
You don't have to look far:
"Plan your circuit using the best
QNH, for example a nearby aerodrome
or failing that the most recent
regional QNH. "
from CAA Safety sense leaflet 12 section 5f
I really don't see what the big issue is. The phrase
Regional QNH offers little scope for ambiguity (it's a QNH-like value), unlike
Regional Pressure Setting which doesn't tell you much about the datum you're setting.
US forecasters don't seem to have any qualms about using
QNH to refer to a forecast value:
EGUL COR 051111 16012G18KT 9999 FEW022 SCT090 SCT130 BKN200 510005 QNH2965INS T19/13Z T12/04Z BECMG 1718 15010G15KT ...(remainder of long TAF snipped)
That might well be WMO-standard.