And as soon as you have "a provider" as a business entity, UK government can ban them from providing E2EE solutions to Apple users the same way they did ban Apple. Or the provider would just silently bend over hand hand all the keys to the UK govt.
They can't. But they can police any service that has substantial number of users. And that's what most of the people would use. So, the hardened criminals would use their own underground darknet services which the government couldn't breach, but the regular people would have no privacy at all.
> 4chan, rejecting their assertions on jurisdiction is certainly an option.
4chan can tell UK regulators to take hike because 4chan has no business presence in the UK. Any service that does want to serve UK users and is successful in doing so, will eventually find itself in UK regulators' crosshairs. For services that are based outside UK, they'd just stop serving UK users because that's the easiest way to handle it. Which is completely fine with UK regulators, in fact, that's exactly what they want - so that nobody would be able to provide privacy to UK subjects.
They can't police every online server you can possibly rent, and they can't police them "all at once" like they can with the Google/Apple duopoly, all they can do is go after them one-by-one as they need access and as we see with 4chan, rejecting their assertions on jurisdiction is certainly an option.