Eh?
DPE (1918) implements GDPR, so what's the issue? Of all the myriad interconnections between the UK and the EU that could cause chaos, data flows are ,well, not bottom of the list perhaps but I can't think of many things less likely to feel some impacts. I forget the exact terminology but even pre-GDPR, transfer of PII outside the EEA was perfectly OK as long as the destination country was adjudged to have "equivalent legal protections" or words to that effect -- similar data protection and privacy laws, and working institutions and processes to enforce them. There's a short list of countries passing the test, .. ah, here we are:
https://gdpr-info.eu/issues/third-countries/
Seems pretty obvious the UK goes straight on that list, and that any reasonable lawyer* at an organisation that's party to data transfers to/from UK jurisdiction would see that there's obviously no issue.
Or they could, you know, stop AD sync'ing between the firms in the UK that are part of operations that also exist in the EU27 - just like they don't have shared AD infrastructures that cross the borders of the EEA today.
:-|
* yes, I know
Biting the hand that feeds IT
