Search for London postcode returns no results

Postcode data in OSM is sparse, at best. This is because most OSM data is gathered by people either looking at aerial imagery or going out in the field looking at reality, and postcodes simply do not lend themselves to being mapped this way.

Commercial operators, like Google, can buy data sets from those who maintain them, such as the Royal Mail in the UK. OSM does not have the budget for this and, more importantly, one of the points (some would say the point) of OSM is to be freely distributable, and the licensing agreements for bought postcode data would almost certainly not allow that.

Well, it appears that no one has ever bothered to input any data for it into OpenStreetMap. See for yourself: Database query for E12 5PG. In fact very few postcodes have tagged at all in your area of interest: Database query for any postcode. Click on the colored circles/shapes to see the postcode listed. None of those in the database match yours.

Additionally, I believe that the search engine on openstreetmap.org only returns postcode areas (if they are mapped). It does not identify objects just based on postcode.

So it seems you would need to map any features with this postcode in OpenStreetMap yourself. That's how the map grows.

It is odd that this postcode does not return a result. Nominatim does use a complete dataset of postcodes for Great Britain which are stored separately from OSM data. I believe the source is either ONS postcodes of Ordnance Suryey Code Point Open. Both data sets are open data, although Code Point Open had certain restrictions associated with it.

If a postcode is not present in the OSM data (and at present we have perhaps 150k or a little under 10% of the total), Nominatim used to display something like E12 5## and the viewport will be centred on the postcode centroid with a medium zoom level; now it shows the postcode at a higher zoom level (e.g. E12 5PQ).

As the specific postcode is present in the ONS dataset, I can only assume that this specific postcode was omitted from the imported data. I don't believe this list is regularly updated. I would suggest filing a specific bug report on github.

Summary (if above is tldr): the postcode should be detected by Nominatim even though OSM coverage is sparse, because it holds a table of British postcodes from an external source.

1: https://www.openstreetmap.org/search?query=E12 5PQ