Postcodes Centroids from Code Point Open data
There is a site to display the UK postcode centroids as an overlay. You can see a web page with the overlay on it at http://codepoint.raggedred.net. You can use this in the Potlatch 2 or JOSM see below. The overlay could be used to help identify the postcode of an object such as a building. Please do not add the centroids themselves to the map. Blog info here: Open Maps
The overlay uses data from the OS OpenData first released April 2010. The overlay was last updated with data released in November 2011.
JOSM, iD & Potlatch 2
See the instructions at http://codepoint.raggedred.net/