IMHO Apple is actually being honest here. They cannot legally operate in the UK without providing a back door, so they are dropping the claim of ADP in the UK. This is letting the user know what's up, and might also help inspire a backlash against these laws. Apple needs to make it clear that they are being forced by UK law to degrade service.

Corporations can't really resist governments unless they're not operating in a given government's jurisdiction and therefore have nothing to lose. They can take things to court, but in lieu of a verdict or an injunction they have to comply with the law or they can be fined, have assets frozen, be de-banked or banned from processing payments, etc.

I'm sure there's services out there that will secretly comply and still claim to be secure.

There's also a lot of companies that will simply abandon security features like ADP or never develop them. Apple is going to the trouble of disabling it only for UK people not everyone, instead of just deprecating it. The latter would be less expensive and expose them to less legal risk.

If you really want security in the UK now you have to roll your own and do the encryption yourself. Honestly that's always the best security, since you can never be 100% sure a closed cloud or software vendor isn't messing with you.