Are full UK postcodes to appear on OSM, or should it be done via freethepostcode or npemap?
The freethepostcode project is a sister project to openstreetmap, so you put them in there.
but there is a postcode POI in OSM (via Potlach) - should I not be using this then? the ones I added to freethepostcode are not showing in OSM and were added well over a year ago!
I add them to both.
Yes, add them to both.
You can add them to OSM using the tag postal_code=EC3A 7PT or addr:post_code=EC3A 7PT
There’s a visualisation of them here: