Sadly it isn't about the "pension" or "forced retirement" - it goes far further than that. I would certainly agree that it is morally wrong for BA (any employer) to do this. But why use UK courts to fight it when the employees are based in Hong Kong, reside in Hong Kong and have no connection to the UK at all (the answer to that is in other posts above this one).
"Unite, the largest union in the country, will be arguing that the airline must apply UK law to its Hong Kong-based female cabin crew"