Another good source of UK Postcode data is doogal.co.uk. The author (Chris Bell) combines the Codepoint Open dataset with other ones from the ONS; that gives a more comprehensive list of postcodes (including ones that are no-longer in use and codes in Northern Ireland and the Isle of Man).

In his datasets he's already done the conversion to regular lat/lon.

http://www.doogal.co.uk/UKPostcodes.php