The UK has unusually detailed postal location codes. There are about 2 million unique postcodes in use at any one time. Each code represents a business or a couple of handfuls of household delivery locations. The overall data is updated frequently as old locations are removed from use and new codes are issues because new housing are businesses are created.
What is the best source for the geographic locations (e.g. the lat/lon centroids) of those postcodes? What restrictions apply to their use?