I mean they could have tried not complying, and fighting a lawsuit at the ECHR (right of every person to a private life). Takes money and time but more attractive than the other options.

It's less attractive, riskier, and more costly of a decision for Apple. Apple is a corporation, not an altruist.

This play by Apple applies pressure to the UK government indirectly via its citizens, for free, rather than taking the risk and expenses of a lawsuit.