I've used https://postcodes.io/ https://postcodes.io/ on a number of projects. There's information about it here - https://postcodes.io/about https://postcodes

I've used https://postcodes.io/ https://postcodes.io/ on a number of projects. There's information about it here - https://postcodes.io/about https://postcodes.io/about - which doesn't fully answer where the data comes from, although it mentions OS, so I presume it's based on the OS AddressBase product? I also wonder how complete it is now, although the sites we've built haven't had any issues as far as I'm aware.