The state of the UK is one of global import, and if they force this technology to be developed and implemented, then not only will the US be a single decision away from implementing the same tyrannical statutes, but the government will absolutely pressure tech companies to adopt it stateside and to give them access. The UK is essentially a rogue nuclear state dragging down the entire West right now, and it needs to be dealt with accordingly. NATO attacked Bosnia, and initiated regime change, for far, far, far less than what the UK has been doing for years.
The UK government spyware demand means that the government decides exactly what should be censored on every mobile device. They say they will start with nude pictures (if you don’t identify yourself as an adult). But it could at any time be expanded to anything the government disapproves of. Today, 30 people are arrested every day in the United Kingdom for writing something online that the government classifies as "grossly offensive". It is obvious that they will use this tool to restrict free speech.
Currently, there appears to be no requirement to report findings outside the device. However, with both legal and technological decision-making power taken away from individuals and transferred to the government, that is only a pen stroke away.
This means that the government could also use this system for total mass surveillance.
And they can do so in secret.
The government recently, in secret, tried to pressure Apple (which is now agreeing to client-side scanning) to build backdoors into its end-to-end encrypted cloud service. They can do this under the Investigatory Powers Act 2016, also known as the "Snoopers' Charter" – a law that makes it illegal for tech companies to disclose secret demands from the government.