What you describe is the Regional QNH, not the local /airfield QNH.
Regional QNH is forecast and published in advance and is used for en-route terrain clearance, whilst local/airfield QNH is measured at the time and is set prior to making an approach.
No-one should try to make an instrument approach on regional QNH.
Even if they could find someone to pass them one
They shouldn't be made on the Regional Pressure Setting either, which is much easier to get hold of
