If the intelligence agencies don't know that their own tools can and will be used against them (and all the data on their own citizens they've kindly gathered for their adversaries) they are willfully ignorant. No excuses.
Walk down any street in the UK you see multiple CCTV cams watching you. Buy something at the supermarket and you'll have at least 1 or 2 facial recognition cams put in your face. The police routinely deploy facial recognition software on random passers by in the street and monitor our social media posts to ensure no one is saying anything hurtful. On entering the UK border police can detain you without grounds and force you to answer their questions (you have no right to silence). They can also force you to provide them with access to your personal devices and if you don't cooperate you'll be charged as a terrorist. They can (and do) legally use this power for arbitrary reasons all the time.
In light of this the idea that the UK would be concerned that someone's iPhone data isn't kept completely private is absurd. This just isn't a concern of the UK state. The concern is that people currently have too much privacy and we cannot be trusted with that.
China doesn't pose a direct threat to my safety or the safety of my love ones, but the government does.
The right to privacy is important because it protects individuals from unfair treatment from all governments. This obviously means that some crimes are harder to solve and in grant that in some cases there is a line to draw. However, in this case the reasoning for eroding privacy doesn't seem legitimate.
For example, the protect the children justification for this is literally absurd when you consider over the last few decades tens of thousands of British children have been groomed and raped in the open and while the police was fully aware. This just wouldn't be the priority if you were genuinely concerned about sexual crimes against children.