Have you seen the proposals the EU came up with concerning .eu domain registrations? If the same panel that gets to decide GDPR adequacy is the one making decisions about data protection then you can pretty much guarantee that a zero tolerance stance will be taken to the UK post-brexit.
The UK seems to be wilfully heading towards crashing out of the EU in a manner that would be incredibly damaging for both sides. This isn't going to help the case for the UK to be given any sort of special treatment in the aftermath, if anything it will harden attitudes.
The USA was given a special pass for data protection because nearly all of the worlds social media companies originated and run their services from there. EU consumers wanted access to that, and were not going to take it kindly if EU regulators shut everything down after these services had already gained traction, regardless of it being for their own protection. Hence a huge amount of kicking problems under the carpet to come up with safe harbor, and subsequently privacy shield. Chance of UK gaining similar status? About the same as the number of international social media companies that started here - zero.
Biting the hand that feeds IT
