Another chunk of data provided for Show Us A Better Way was the Postcode Address File from the Royal Mail. This isn't the Postzon data (which has postcode → latitude/longitude information), just a big table of address data for every UK postcode (30 million of them).
It's the kind of data, apparently, that you have to print out, sign and return a form to the Royal Mail before they'll send you a CD by post that contains 350MB of data split into 14 separate zip files. Unzipped, the 'Compressed Standard File' is 7.5GB and most of that is padding - the PAF data is in an archaic format where each record is on one line and each field is a set length. Once you finally get it into MySQL (here's the script I used for parsing the files), it's about 2GB.
Here's one use of the data: the 1000 most popular house names in the UK (not including Wales because that data's in a separate file).