I don’t know how this would even be possible.
What they are saying is that they want an entirely new version of ADP for British users right? Not a backdoor that only works for British users? That latter would be a de-facto backdoor to all users regardless of jurisdiction.
Even if they create an entirely new version of ADP for British Apple IDs, will it even be useful for them? An easy workaround would be to switch your account to another country and use a VPN on top of that. I’m sure that only evil terrorists and predators are using ADP to store their files. Totally not regular civilians.
This is clearly a mass surveillance play by the current British government.