I guess the devil will be in the detail of what's actually in the pay-awards agreed by the unions - something we don't know (I guess it's not in the public domain).
First of all, my guess is that it is the middle managers and lower senior managers who are the UNITE members who are benefitting from this bonus scheme... I would be surprised if the most senior managers are union members - and if they are, I doubt that the unions would be directly involved in negotiation of their remuneration packages. In my experience of a different big company, I expect the 'big bosses' agreed the incentive package at board level - and this was cascaded down in the form of goals and objectives to an array of different departments and individual managers. So by the time incentive schemes have filtered down into the measurable objectives at those levels, they'll have been 'sanitised' to an extent. (Perhaps the 'divide and rule' comment is appropriate here)
I find it highly unlikely that UNITE would be negotiating pay award schemes that gave incentives to managers based on explicit actions to impose unagreed rules on other staff. Far more likely that the union negotiated a basic framework for the bonus scheme. IMO, at most, UNITE negotiators MIGHT have been aware that bonuses might be achieved by delivering certain headcount reductions, introduction of certain new working practices and X% efficiency savings etc.
It's a manager's job to identify and deliver such efficiency improvements, so UNITE couldn't be blamed if they allowed their members to accept such a bonus scheme. The problem is then HOW those 'efficiencies' are achieved - but that's (probably) another matter.
There is always something of a conflict where middle and relatively senior managers are union members. It's perfectly reasonable for them to be union members because they are themselves just employees. But the potential for some conflict is clear.