A BA director is trying to coerce and employee to leave a union
No one who has read the ESS mail detailing the new offer could suggest he's doing anything of the sort.
Indeed rather the opposit, if you were a member of the union the offer isnt available to you to decide on individually as you have choosen to let your reps have that power, even if you left the moment you recieved the mail, too late.
No benefit in leaving, therefore no encouragement to do so on the part of BA. To have done so would have indeed have been unlawful - which is why they didnt.