First order and download the Code Point Open files from the Ordnance Survey and unpack them locally. You should get a Data/CSV directory containing a file for each postcode prefix. In total there were about 1.68 million rows (Q1 2013).
Create your database and install PostGIS extensions (assuming Postgres 9.1+, PostGIS 2+, and sufficient DB privileges).
$ createdb geopostcoder
$ psql -c "CREATE EXTENSION postgis" geopostcoder
Then you can load the Code-Point Open data into your db:
$ cat /cpo-download/Data/CSV/*.csv | script/load_code_point_open -d <database>
copy config/database.yml.example to config/database.yml, and edit it to
match your database name
Install the necessary gems by running bundle install, then start the server
with thin start.
Point your browser at http://localhost:3000, and try out reverse geocoding some latitudes and longitudes to UK postcodes.
GeoPostCoder is released under the MIT License.