Dr P,
There's nothing wrong with using a METAR QNH for an approach. You just can't use it for the 100' reduction to minimas.
Terminal 5.3 in the Jepps also clearly states that you can't use METAR QNH for the purposes of this minima reduction, regardless of the fact that it may be less than 15 minutes old. It's still a METAR isn't it.
If an approved observer gave you the conditions over the radio or as you said, as you were stepping for a flight, then you would be receiving an observation of actual conditions, the same as an awib. But not a METAR.