Unfortunately, though, the database of postcodes and their locations is another victim of the British institutional custom of copyrighting taxpayer-funded databases and licensing them only at great expense and under onerous terms (see also: the Ordnance Survey), effectively restricting them to moneyed corporations. However, there are several unofficial efforts to assemble this data from scratch and release it into the public domain.
Of course, if one were to use the leaked data in any public way, one would be sued for violating Crown Copyright and would almost certainly lose, which makes it little more than a distraction.
A boycott of the postcode, when sending letters, has been suggested (not by me but on the Register at http://www.theregister.co.uk/2009/10/06/royal_mail_ernest_marples_postcodes/ )
I didn't realise this was the sort of thing that wikileaks wanted to get involved in (it's not something that suppressed because an organisation doesn't want people to know about it, but something that's suppressed because they want people to pay for it) but someone put it there anyway:
http://wikileaks.org/wiki/UK_government_database_of_all_1%2C841%2C177_post_codes_together_with_precise_geographic_coordinates_and_other_information%2C_8_Jul_2009