Pugwash 4x4: unions are mandated to protect their members interests as far as possible - not the interests of those who may or may not choose to join the company. FWIW the affect on Trainee salaries was made very clear to the management team at the time, & they chose to ignore it: they realised they'd gone too far quite quickly afterwards, hence the "living allowance" payment. Your argument is with them, not the union, and by joining NATS anyway, you strengthen their case, & weaken the unions. If we ever reach the point where recruitment can't get people because the salary is too low, the policy will no doubt change again - but no sign yet.
This battle was won/lost at least three years ago - surely you've had internet access since then? There's nothing to stop you changing your mind, even at this late stage.