How does OpenStreetMap solve it? OSM, more specifically OSM Nominatim, shows postcodes. Example: https://www.openstreetmap.org/search?lat=53.151778&lon=-1.16...

I didn't know the answer so I looked it up. Nominatim gets postcodes from Ordnance Survey: https://nominatim.org/release-docs/3.4/data-sources/GB-Postc...

Specifically Code-Point Open: https://www.ordnancesurvey.co.uk/products/code-point-open which is updated quarterly, and in turn gets the postcodes from Royal Mail.

It seems to me that you can download the postcode list freely: https://osdatahub.os.uk/downloads/open/CodePointOpen

Something is missing here. If OS already has that data from RM and can make it available freely, why would they need to build another database?

Because the OS data doesn't provide addresses, just locations of the postcodes in coordinate terms, so you can't provide the typical website address lookup.

I can't find any good information post-privatisation, but at least before 2013 the postcodes themselves were copyrighted by Royal Mail (likely Crown Copyright as with government data). There were attempts to enforce this in 2009[0]. I suspect the copyright is now owned by Royal Mail Group Ltd.

That aside, a practical issue is that Royal Mail still retains the rights to _allocate_ new postcodes for any new properties. Yet another failure of this particular privatisation.

[0]: https://www.techdirt.com/2009/10/06/uk-royal-mail-uses-copyr...