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But there are open alternatives that OpenStreetMap draws on.

http://wiki.openstreetmap.org/wiki/WikiProject_UK_Postcodes

Maps.me now lets you edit OpenStreetMap directly, so you should be able to add your own postcode directly. I don't know what kind of UI they've added, this can get kind of deep, but nothing that someone reading HN can't figure out.

> Update 2010-05-10: Post code data from OS now freed

Postcode centroids have been freely available since 2010. freethepostcode was set up before then (as was npemap, a similar site I was involved with: http://www.npemap.org.uk/FAQ.html#whatIsIt ).

Postcode boundaries, however, are still not available openly.

I wonder how much absolute error there is between the actual boundary and a Voronoi boundary. While I'm sure there's some massive IV postcode for which it wouldn't work, if most postcodes are tiny, then the fact that you're off by under a kilometre isn't so bad for routing.