Is it? 
Let’s not confuse the law and morality here lol
I don’t think it’s known whether Apple has taken any action at all, but I doubt they have. The order was only issued in January.
To me this seems like another example of a government trying to act tough that will ultimately not really go anywhere and quietly go away. It isn’t a reasonable request to stand by.
Not only PR. Think about it this way, if the UK gets a backdoor into iPhones worldwide, what government agency anywhere else in the world in their right mind won’t immediately ban the use of iPhones? Everyone is paranoid about Chinese spies and Russian spies, but they’ll be just as paranoid about UK spies if they had this power, no doubt about that.
Apple would lose more sales if they stayed in the UK and added this backdoor, than if they dropped the entire UK market. I’d basically guarantee it.