New Public Edition maps are trying to create a freely usable UK postcode database. The British Post Office owns the database of postcodes and their corresponding coordinates. That means that your website can only use post-codes if you buy a license from the Post Office.
New Public Edition (along with a similar project, Free the Postcode) is trying to solve this. They have 1950s-era public-domain maps and they ask you to locate your house (or childhood home) on it and key in your post-code. They do the rest, eventually building out a complete database of every postcode in Britain.
The resulting data will be released as purely public domain--no restrictions whatsoever on re-use.
I just filled in a couple of postcodes from previous residences, and it was quite interesting to see how the area that I grew up in has changed since 1950; it looks like it used to be one large farm that was broken up into a couple of smaller farms that have now become residential areas. It's a logical progression really, but having a date like 1950 gives a sense of dimension--it's easy to think that that change happened "hundreds of years ago", but it's much more recent than that.
So, if you're in the UK, or lived there for a while, please take a couple of minutes to visit New Public Edition, fill in your postcode, and perhaps gain a better understanding of the places you've lived.