I completely fail to see what is so important about this. Don't you by definition need to know the postcode of whomever you need to send mail to? If so, what benefit do you get from this stuff?
A lot of stuff that is paid for by taxes in the UK is not automatically public domain, but copyright of the government (or technically, Her Majesty's Stationary Office, the govt. printing bureau) and only available on payment of a stiff fee. Basically the state qua legal entity is much more 'selfish' than in the US, where pretty much anything it puts out is public domain by definition, and it's deeply annoying if you're the sort of UK resident with a presumption of that information wants to be free.
We licensed the Postal Address File (the list of official correct addresses for each house) and we had to add encryption to our product to stop people being able to extract the raw data from the code.
The same applies to Ordnance Survey map data - there are a lot of apps that aren't possible/easy in the UK because of the cost and licensing of the maps that the taxpayer paid for.
Usually postal code data isn't that accurate though. The Long/Lat information is usually just a point in the 'center' of the postal code and most postal codes are irregularly shaped.