Obviously the EU is a force against that, standardizing regulations across the whole EU. Facebook is changing how it relates to UK users because they have left the EU, of course.

At the moment there aren't really any significant contradictory laws/regulations I am aware of. Facebook could just choose to treat everyone as if they were protected by EU laws, but they choose not to. They didn't have to change how they relate to UK users, but they legally could and they chose to.