The way this has gone, I would strongly favour laws that restrict the use of industrial action to matters which could or do directly affect the current role of the employee or employees who are withdrawing their labour. So in this case, a strike would not be possible if Northern gave a guarantee of employment as a guard for any present member of staff presently holding that role who wished it, with the option of course of "buying out" those who might take a bit extra money, annual leave or whatever to become something else instead, be that OBS, driver or whatever, or to offer genuinely voluntary redundancy/early retirement to anyone who was willing, by choice and without coercion, to take such. Of course, Northern are not presently offering this, but perhaps with this kind of change in the law it might be in their interests to do so. If they did, to me that's the problem over.
The exception to this would be where the Union felt a particular member of current staff had been treated unfairly, e.g. sacked without adequate grounds.
Unions should not in my view be allowed to dictate or seek by way of industrial action to influence future Company policy where it does not affect the current role of members of staff being balloted or proposing to strike, regardless of the grounds for wishing to do so.