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There is a pretty good effort to reverse engineer it all:

http://www.freethepostcode.org/

Edit: actual dataset here: http://www.freepostcodes.org.uk/ - link above wrong.

It doesn't look like it's been updated in a while, and many of the links are busted :(

Sorry wrong site. Have updated link!

It plagued and still plagues. The Royal Mail is ruthless when it comes to IP of UK's post codes (read US zip code). Every "little guy" who dare tryed come up with ways around it were and still are crushed with C&D or plain old take downs followed be a gagging order. Myself included.

Any more insight in to your case?

Here's another list of Zipcodes:

http://www.zippopotam.us