I personally think it should be a "yes" vote received from 50% +1 of the entire workforce who are eligible to join the union whether they have actually joined the union,
When you say workforce, do you mean the specific group, or the whole airline?
Further, there seems something very wrong to me in union leaders being able to advise members to vote in favour of strike action even though they don't intend to strike.
But is it ok when the same union leaders recommend acceptance of a deal?
There appears to be a tremendous imbalance as the employer has to be squeaky clean in everything it does (e.g. not inducing union members to resign, etc.) whereas the union can behave as an openly dishonest bunch of renegades while being afforded incredible protection and immunity from the liabilities arising in the substantial damages it can inflict on a very broad group of completely innocent people.
That depends if you type as a manager or as a TU rep. So far more reps have been sacked or suspended, than managers.