British Telecom has published the complete UK phone directory online. As there has been so little publicity about this move, it is presumably still under test, but the service is already listed on the BT website for public use.
Users enter details of the required name, plus optional information about any part of the address and whether it is a private or business line.
Using local rate calls, this service will inevitably be cheaper than dialing Directory Enquiries for the same information.