UK Postcodes
Free online address search from a postcode. Print a label & view on a map.
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UK postcode free API integration scripts
Free UK postcode databases & example integration scripts for PHP, .NET, JSON, AJAX etc.
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UK postcode free API integration scripts

If you need full address look-up on your website then this list of UK providers will be useful.
Postcode API providers

UK postcode free API integration scripts