The "zip" and "zip" get a little confusing.Go here:
http://www.tomtom.com/updates/zipcodes/Map_850/zip.zip
and get the 850 version of postal codes for the UK. Unzip them into the zip folder on your unit.
Read fine to me, although this must take the record for the most number of zips in a single post - 13 in total - MikeNot sure that last step was clear.