Striking is a legitimate tool to be used when a dispute cannot be solved.
It seems to me this dispute could easily have been solved, once BA's offer was received, between the two parties affected: BA Cabin Crew and BA.
The fact that certain idiots in BASSA and UNITE have paid no heed to the interests or desires of their members and have royally
things up, you can argue that they are now calling a strike which is legal, but good luck trying to convince people that it's "legitimate".