So, The union doesn't doesn't like imposition, but when BA negotiated and tabled a proposal that could have been accepted it was dismissed out of hand
To BASSA its imposition, in any other department of BA or any other company it would be called managing your employees.
Unions are necessary to protect poor downtrodden employees. In this case did the cabin crew really need defending against a malevolent employer whose original offer they would now love to accept.