The postcode-to-coordinate data is now freely available as "CodePoint Open"

So there's already data for people who want to know postcode AB10 1JL corresponds to the area around 57.14677,-2.09873

The PAF is a more detailed data source, as seen in https://www.royalmail.com/find-a-postcode which can tell you that AB10 1JL specifically covers the addresses

102-104, Union Street, Aberdeen

82, Union Street, Aberdeen

Timpson Shoe Repairs Ltd, 86 Union Street, Aberdeen

Smart Mobile, 88 Union Street, Aberdeen

92 Union Street, Aberdeen

98 Union Street, Aberdeen

The PAF is useful if you want to provide a "quick address entry" option on your website - and to validate address data. But if you just want postcode-to-location conversion, that info is already available.

Given that OSM has a lot of building addresses and locations (at least it has building names and numbers and the streets they are on, plus their outlines) I wonder how much of the PAF could be recreated from the data in CodePoint Open combined with OSM. If adding more house numbers etc to OSM would help improve that then it could be crowdsourced quite quickly via Street-Complete or similar.

For the purposes of the quick address lookup it doesn't even need to be 100% accurate. If a property is incorrectly listed against two postcodes it won't really cause problems, so the lookup could err on the generous side

Thanks for the clarification. I’d forgotten that ernestmarples only offered postcode-to-location lookup: it was a long time ago.

I suppose this is encouraging! It shows that the forces of openness are gaining ground in this battle.