I don’t think this is correct. EU is pretty strong in favour of data localisation, UK is not. Moving customers to California doesn’t mean they are no longer under UK privacy rules, which of course are identical to EU privacy rules (for the foreseeable future, could change of course).

While I agree with the theory of what your saying, I'm not so sure it matches the reality.

If it did, then why is Facebook moving the data? There would be no benefit. Also it's currently not clear that it's actually legal to move EU citizen data to the US, because the EU doesn't thing the US has strong enough privacy protections.

I know that EU citizen data is in the US, and people keep coming up with clever legal structures to make it legal. But those structures keep getting ruled illegal by EU courts.

Because they don't own a datacenter in the UK and it probably doesn't make sense to open one for a country that small

So the next best option is to move it to the US based data centers

It's not about localisation of data, it's about localisation of control of those data.

The EU has been one of the strongest pressure points towards forcing the rest of the world to acquiesce to EU specific requirements or leave. As such, the EU is at the forefront of balkanization of the internet.