Do not try to solve legal issues with technology. They're not the same scope.
If the UK wants Apple to introduce a backdoor, the two options are to do it or not to do it. To not do it and invent some excuse leaning on technical details doesn't work. Apple has the keys to sign software for these devices at the end of the day. That includes software that betrays the user's trust. That previously data was incidentally stored unencrypted on servers is not relevant for the fact they're being compelled to make this data available - at least not with my understanding of UK law.
If the UK wants Apple to introduce a backdoor, the two options are to do it or not to do it. To not do it and invent some excuse leaning on technical details doesn't work. Apple has the keys to sign software for these devices at the end of the day. That includes software that betrays the user's trust. That previously data was incidentally stored unencrypted on servers is not relevant for the fact they're being compelled to make this data available - at least not with my understanding of UK law.