Unified Software's AddressVal API allows users to provide a partial address, and returns one or more addresses from the Royal Mail Postcode Address File. If AddressVal can find an exact match, a single full address will be returned. Otherwise, a number of possible matches will be provided.
This API can be used via either SOAP or REST. Access to the API is dependent on a username and PIN that can only be acquired by contracting a sales representative of Unified Software, so it is possible that the service is restricted to paying customers.