I suspect that even though the first two strikes were not properly called, and so strikers continued employment was formally not protected, any one dismissed because they struck would have a reasonable chance at a tribunal to claim that they acted in good faith, based on information they were entitled to believe that the strike was properly established.
Sadly, ignorance or good faith don't have much weight in a court of law. If the March strike is deemed illegal (and that is still a big IF) then the employees who went on strike are out on a very precarious limb.