Address lookup or just validation?
For validation, there's an official regex (archived, but still useful) http://www.cabinetoffice.gov.uk/govtalk/schemasstandards/e-g....
For address lookup there are a couple of ways to go about it.
1. You can buy the rights to the Royal Mail's database. You get to parse the sucker yourself. http://www.poweredbypaf.com/end-user/products/paf-raw-data/
2. You can use a third party system, either over the web, or choose a solution you can host internally. Depending on the type of system you buy you pay by the lookup or by the user (mostly internally hosted systems).
[1] http://www.ordnancesurvey.co.uk/oswebsite/products/code-poin...
We are hosted on DotCloud(YCS10) - and thanks to generous agreement with them, we have a very large free plan and until we have issues managing demand, we don't plan to ask for donations.
Several well known sites us zippopotam.us such as TheNounProject and Pocket.
There are several alternatives to us and if don't trust us, don't use us - our intention is to serve sites that can't be bothered to download dbs and serve the codes themselves.
Also, to the author(s) of zippopotam.us, if they're reading: - How can we trust you're not going away any time soon? I don't see any pricing, so I assume this is all free? - Related to the first point, I assume you don't want to be used by any high traffic site, since you probably don't have a lot of server power behind this. What's the plan?