Unions of old used to target greedy bosses. They would threaten to put them out of business unless the bosses gave them a decent wage. Unions were an important counter-weight to the employers' cartels that effectively existed in previous centuries. (The option of leaving your job as a dockworker in Portsmouth, for example, for a better paid one simply wasn't an option if all the employers in the area offered indentical terms or told each other about trouble makers.) Those days are long gone.
Now, instead, unions are more likely to be picking on the general public than a private employer. Tube strikes, the annual Heathrow strike, job centre strikes, the Grangemouth strike: all these were aimed at the innocent public. The reasoning from the unions seems to be that by putting enough political pressure on the employer (which is usually government owned or at least highly regulated) new terms can be achieved that simple market forces would never be able to match. For example, tube drivers are paid more than many teachers in London.
The latest example is postal workers, who now are threatening to ruin Christmas for countless children unless they're paid more. And this in an industry made increasingly irrelevant by email and that already offers such poor service that even old-fashioned business such as Christmas cards and presents are being driven elsewhere.
The simple answer is simply to ban strikes that cause 'disproportionate public inconvenience'. It is unacceptable for these dinosaurs to hold us all to ransom.