it will not make changes that it characterises as “a serious and direct threat to data security and information privacy” for users around the globe

That’s the thing right there. If Apple makes a back door for the UK, they would have difficulties keeping only UK citizens affected. Communications are global. Even if they restricted the back door to conversations involving UK residents, every private group chat that resident user enters becomes snoopable just by that user being there and the history could be made available from before the user entered. It would be like a privacy virus with UK residents as the spreaders.