It’s because the proposed changes would give the UK government de-facto authority to dictate how security and encryption are implemented.

…a provision that would give the UK government oversight of security changes to its products, including regular iOS software updates. The Home Office consultation proposes “mandating” operators to notify the home secretary of changes to a service that could have a “negative impact on investigatory powers”.

It would mean in practice that the UK would dictate how Apple employs encryption around the globe, unless Apple was willing to fork their software and build/maintain a UK-only branch for their products.

Which still wouldn’t solve the issue because if you interacted with someone over any of those protocols who was in the UK, your messages and data would be accessible by the UK government, regardless of the other party’s location.

I’m with Apple on this. This isn’t a consumer-focused piece of legislation for repairability/interoperability like some of the newer EU legislation, this is a government trying to ensure they have the technical ability to spy on their citizens and others. It’s the definition of anti-consumer.