I am in the middle of a project for a client and they have asked if it is possible for a postcode search facility very similar to the one on this site: Oven Cleaning UK where the user inputs the first half of their postcode and a page will appear with a franchise/store in their given area.
This is something I’ve never had to do. I always assumed you had to use the expensive Royal Mail PAF or sub-contract to another company that does. Whilst working for a company that was interested in doing postcode analysis on their customer database I went to a seminar at Pitney-Bowes. Their products, which are excellent but very pricey, make doing this sort of stuff relatively easy.
I would have thought Google Maps might be worth investigating as a possibility since they have postcode search and a very good API.
I also found this script which contains both a PHP file which you upload to your server and a SQL file which you upload through phpMyAdmin. Select the database, choose Import then select the file postcodedata.sql and click Go.