So, transferring your data between the EU and UK will mean you are (now) crossing a data border. That's a change.
I understand that GDPR will immediately split into EU GDPR, and UK GDPR.
Something I've seen in lots of UK law derived from EU directives is that it applies limits to behaviour specifically happening in the EU. So UK privacy law might provide a whole load of protections to EU citizens, but not specifically to UK citizens, because EU citizens would be a superset including UK citizen. Except, as of 1st Jan 2021, that won't be true any more.
Kinda makes me suspect that some companies will just ignore UK data protection law on the basis the UK can't do squat to enforce it.
That suggests they've spent 2 years explicitly lobbying for whatever UK law changes they want, and will probably quickly get them.
Even without that, it doesn't make sense for Facebook companies to store the UK data in the EU/Ireland, where it would presumably then have to comply with both EU GDPR and UK GDPR.
> There will be no change to the privacy controls or the services Facebook offers to people in the UK
> Facebook’s UK users will remain subject to UK privacy law, which for now tracks the European Union’s General Data Protection Regulation (GDPR).
Presumably this move in preparation for when that stops being the case.
The UK's position is strange because of Brexit, but it seems the practical position of all recent governments has been that existing EU laws should be transposed into UK national laws where necessary and with the required changes to make sense, until such time as our national authorities might decide to deviate from them. Primary legislation has already been made to that effect. The big question is what "that effect" actually is, given the huge volume of legislation affected and the potential ambiguity in what changes are required for any given law of EU origin to continue to make sense in a post-Brexit UK.