I think there's a terminology misunderstanding. UK uses a regional QNH. Oz uses an area QNH. On the surface they seem the same however there's fundamental difference between them.
UK regional QNH: The lowest forecast QNH for the whole region/area
Oz area QNH: The QNH accurate to within 5 Hpa (Mb) of all QNHs for the area. The predefined AQNH regions/areas are subdivided as necessary to maintain not more than 5Hpa difference.
The upshot is that there is only 150' max difference for a/c on a local/aerodrome QNH vs. area QNH. Accurate enough for traffic comparison. This is not the case for the UK regional QNH. The tolerance is considered sufficient to be acceptable for CTA step penetration avoidance
An area QNH is great for cross country flights because it reduces the number of QNHs that must be sought & set and keeps all the cross country traffic on a common reference. All a/c apart from local/aerodrome traffic use the single AQNH but are still no more than only 150'/5Mb different from the local traffic.
My preference would be for a 100'/3Mb tolerance. Easily do-able in the UK