Still no explicit indemnity from costs
Isn't this covered by the "By joining the list you will not be a party to the action, but you will be entitled to the benefit of the outcome of the case if successful" comment? If signatories are not parties to the action, it is hard to see how they can be made liable for costs.
I am not a lawyer, but my interpretation of the letter is more that if members do not "join the list", then BA will not owe them anything even if Unite wins its action. Basically, if members do not join the list, implicitly they are acknowledging no contractual breach by BA, or at least not one they wish to contest, and therefore no entitlement to restitution in the event Unite wins. This appears to be the point of the "you will be entitled to the benefit of the outcome...if successful". In other words, members who do not sign up will not be entitled to the benefit of the outcome if successful. More importantly, perhaps, Unite's argument is that each and every member's individual contract of employment has been breached by BA. Asking the Union to provide a list of members who wish to assert this position seems reasonable. Or, put another way, if only a small number of members, all of whom hold Union office, assert this position, then it suggests the action is vexatious.
Whether or not BA is in breach is far less obvious than many people on either side of the discussion seem to think. However, it is only an issue anyway if people contest what BA has done. Presumably, the reason the judge asked for a list of people affected by the changes was to test whether or not individual members actually wish to do so.