Two of my wife's team came to see her and apologise for going on strike next week because they knew the team had deliverables that would be affected.
She simply said "You're free to turn up and work - going on strike is not compulsory."
It remains to be seen what happens.
Interestingly, two of her team recently left the IRD and rather than replace them with secondments from within the business she did in fact get contractors in because she knew they would not disrupt the workflow. Funny how unions never grasp the unintended consequences.