I get that they're still working on moving production elsewhere and China can still strong arm them, but I'd like to see them (and other tech players) actually follow through and leave countries that are trying to ruin their ability to provide the bare minimum privacy to their customers. It would be interesting to see how those politicians did if they effectively banned Apple from their country.
The UK's surveillance proposal is more draconian than China's current treatment of Apple, though. FaceTime an iMessage work exactly the same in China as they do in every other country. They're fully end-to-end encrypted and Apple's logging of metadata is extremely minimal. China's policies are deeply problematic they seem content to let Apple get away with the bare minimum of legal compliance, in contrast to local companies who bend over backwards to comply with every whim of the CCP. Could Apple make a principled stand against China? Sure. Would that make some self-righteous people feel good? Definitely. Would it do anything at all to improve the privacy of people in China? Absolutely not. They'd lose their most-private option. That's the real-world outcome.
The UK, on the other hand, is actually still a democracy. A combative and principled stand against government overreach can actually change government policy and preserve end-user privacy.
It's not about security or privacy for Apple. It's about the image of it.
Apple devices aren't inherently more secure than competing devices, they just make a bigger spectacle about it. In this case they could easily region lock the feature being requested but it's much better for their image to just sling threats. I'd honestly not be surprised if they are also developing the feature in the background so they can roll back out to the UK at a later date.