We're looking for a good web service or datasource that provides address verification for the United Kingdom. Was wondering if anyone else had used this type of product and had recommendations?
If you're just after the pure data then you'll need this :
The Postcode Address File (PAF®) is the most up-to-date and complete address database in the UK, containing over 28 million addresses. PAF® is an invaluable tool for creating and maintaining mailing lists and databases, as well as reducing the number of returned or undelivered items.
PAF® is the only complete source of all known UK Postcodes.
If you want a software frontend to it included then there's this. I've not used it for a couple of years but I don't think it had a webservice aspect.
Postcode information in the UK is owned by the Post Office and not free, easily running into tens of thousands depending on your requirements.