Yeah, but have UK privacy laws been updated to reflect fact the UK is no longer part of the EU?

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.

I understand that those laws have been updated: that EU law is initially being adopted with appropriate changes, i.e. it won't reference the EU any more and will reference the UK instead (and isn't defined around citizenship anyway)

I have a similar understanding, what will be interesting is how the UK will enforce its rules. Facebook has its HQ in Ireland, which notable not in the UK. This also applies to most other US companies.

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.

Yes, same. Also, Facebook has Nick Clegg on staff.

That suggests they've spent 2 years explicitly lobbying for whatever UK law changes they want, and will probably quickly get them.

They must have had some work done on figuring out which legislation needs updates like this?

Yes. And that also means protections remain the same regardless of this move from Ireland to US jurisdiction.

Yes, for now, although it would surprise me if changes haven't already been negotiated with the UK government: Deputy Prime Minister Nick Clegg has been on Facebook's staff since 2018.

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.

That’s former Deputy Prime Minister, for the record. Considering his subsequent performance at the polls, a lot of people would further add the prefix “disgraced”...