Well, last time I was in the land of lo pressure (Shetland/ABZ), some time ago admittedly, we had a day whrn the QNH dropped below 950 and some of our (ancient) fleet could not fly as the altimeters would not calibrate below that. On that ocasion those cabs were grounded but there were tales of previous occasions when dispensations had been granted by the CAA to operate with altimeters registering 1000feet high. Can't verify this as fact, maybe someone on Rotorheads would remember if this is correct or my lousy memory or a just another North Sea Myth.
Checked my EFIS kit last week and it went to a scary 750! Some Zebra's @rsehole that would be!