Another angle for the OP......
A TU can only act in defence of its members interests when the members themselves vote for the action. In the end, it is all about POWER.
If the members in, say, Monarch, have a dispute, only Monarch members affected can take any action. If, say, BALPA threatenned action by, say, BA pilots, about a Monarch dispute, the employers would be in Court sharpish, and would win.
In short, collective action is extremely limited in the UK by law. Law that has been endorsed by ALL the political parties.
Anyway, collective action in Aviation is not normally supported by the staff impacted - look at the BA CC dispute. A few hotheads, who backed down and lost due to a lack of support by BA CC, and due to other parts of the same TU - Unite - telling them to shut up and give up.