Knowing every house and having enough friends round the village to ask them their postcodes sounds like a good approach and shouldn’t be legally problematic at all.

The problems potentially arise when people mass-copy postcodes into OSM, or state that they’ve copied postcodes from the Royal Mail database. At that point, if the Royal Mail were to notice they’d have good grounds for asserting their database rights. At best, that would mean volunteer time gets consumed in erasing the unlicensed data from OSM. I believe they have taken action against people other than OSM copying their database before (they make money by licensing it to businesses), so it’s not a baseless threat. It is unlikely though.

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Thanks for testing StreetComplete. It’s a shame it isn’t fully compatible with the format from the wiki.

I think my inclination would still be to use a combination of the first and third formats I suggested. That would be compatible with the wiki (most of its advice around addr:place is to avoid using addr:place and addr:street together), and would keep StreetComplete happy.

While you may remember to not answer the street/place StreetComplete quests, other users in future will not, so going with addr:city with no addr:street is likely to get messed up the next time a well-meaning user who hasn’t read this discussion does some StreetCompleting in Casterton.