Diplome
I will try this from another angle. BA are encouraging employees who are members of Unite to vote in the current ballot. OK, they are suggesting that the only way that a current Unite member can enjoy the benefits of the new deal, is to be part of a rejection of the call for industrial action.
The excellent work by BA in making the ballot a vote on the current offer, rather than for industrial action and not just rejecting the ballot as potentially unprotected, implies that it will be seen as a sound and lawful process.
As to gross misconduct, I would say any reference to encouragement from the reps from what I have read, is to vote and it would be reasonable to expect one of the activities protected in law would be to support the call for action by the union that you are a representative of.
It still doesn't stop BA sacking who they want however, if it serves a purpose. The gross misconduct charge is just to stop any compensation award. However, is discrimination against an employee for union activities an uncapped award? Future loss could take the award into the millions.