[moring]

It seems to me that you can download the postcode list freely: https://osdatahub.os.uk/downloads/open/CodePointOpen

Something is missing here. If OS already has that data from RM and can make it available freely, why would they need to build another database?

[sahmeepee]

Because the OS data doesn't provide addresses, just locations of the postcodes in coordinate terms, so you can't provide the typical website address lookup.