OK... Depends what format you want this and if you need the complete postcode system etc.
The complete data is available from the Post Office in the UK and normally, or was, on a subscription basis where they periodically provided changes/updates. This was provided a record per line together with the corresponding town and county. I used to write software for direct marketing/mail order and used this format which allows loads more flexibility, tho can be a hassle if the postal service does not produce the correct data asap. If you want the raw data it's available from the Post Office - ask for the PAF (Postal Address File) CD. It used to cost money, but not sure of the current prices.
Alternatively there are other software houses who specialise in playing games with the PAF data and offering utlities to manipluate and use this in other software. Here's probably the main developer of "plugins" from the PAF:
http://www.afd.co.uk