What I don't understand is why Apple is even responding to this absurd demand. Completely banning Apple products in the UK isn't a realistic option for the government, so Apple could simply state openly that it does not cooperate with authoritarian regimes and actively prompt British users via a pop-up to enable ADP to protect themselves from the government.
It seems you are suggesting that if a private corporation (a foreign one at that) is large enough it should ignore the law of the country its operating in. I don't think you will like the consequences of such a situation, and will charitably assume you have just not thought your message through for even a second before posting it.
PS: If you think the British gov is authoritarian.. well, you might not know what the term means and/or anything about the spectrum of governments operating today or in the past.
But apple also cannot just stop selling or supporting their product in the UK either. So that card is dropped from both sides. As long as apple is beholden to a few million costumers and subscribers to their services in the UK; UK law is able to pressure them, fine them and restrict them because of those users.
We don't want it the other way around; companies placed outside the borders doing and selling whatever they want without a care. So we end up in a negotiation.
Apple choose to maliciously comply; subtly revealing the TCN, giving all UK users notices about what their government demanded of them to encourage public outrage, and push on the correct parts of the UK government that has their interest in mind.