Postal codes in the United Kingdom
Most addresses in the UK can be identified by house number and postcode, but the postcode database is not fully available for free.
Postcode centroids
See:
'Special' postcodes
Certain organisations such as banks, medical centres & hotels were given unique post codes, presumably due to the high volume of correspondence they'd receive. Even though many of the properties have change businesses over the years (especially banks becoming eateries) they still retain their individual codes.
If using the very undetailed code-point_open you cannot assume that a restaurant in a large building with unusually high ceilings has the same code as it's neighbours. Local, on the round knowledge usurps the database.
Postboxes
The postcode project most closely involved with OSM is the Postbox locator on dracos.co.uk. This uses a list of all post boxes (with the postcode area they serve) supplied by the Royal Mail and tries to map them onto OSM data. You can help by adding the 'ref' when you add a post box. See Tag:amenity=post_box for details.
Historic projects
Prior to the release of postcode centroids there were a number of projects which aimed to collect postcode data by user contribution. These were:
- Free the Postcode which asked you to enter your postcode and GPS coordinates.
- npemap which let you find your postcode on a scanned out-of-copyright map.
You can read more about Free The Postcode and see an old snapshot of Free The Postcode UK Coverage.
Some data from these two collections was imported into OSM using the uk_postcode_centroid=* tag (stats) on a node that is roughly the centre of a postcode prefix area (for example uk_postcode_centroid=SW1).
There was also a map of postcodes and a table of UK towns by postcode, put together by the OSM wiki community.