If Bassa/Unite vote to reject the proposed contract can/will BA offer any CC who elect to resign from Bassa/Unite after 6/25 the opportunity to sign the contract?
Probably, the date was chosen as they cannot legally "induce" people to resign from a trades union.
However, BASSA are really really bad at deleting resigned members so pretty much anyone could claim to have resigned before the 25th and BASSA would have the devils own job proving they hadnt.