Apparently, the electricity measurement in Vaillant HPs is MID approved: https://x.com/_heatgeek/status/1752791102850252902?s=20
The inbuilt meter is being used for the OVO Heat Pump Plus trail for billing purposes via the Vaillant Internet gateway.
The heat output measuring is not MID approved, it seems to be the temperature sensing is where the greatest error and variability is found.