If crew work for long enough under the new arrangements they will be deemed to have accepted them. (or does the calling of a strike ballot invalidate that argument, counting as a protest against the new terms?)
Not sure if calling a strike ballot would count as working under protest, usually one would have to write to the employer as an individual asserting the right to continue working under protest whilst claiming unfair dismissal from the old contract. Maybe Unite's legal team know different.
Having recently left BA myself on VR, I am an example of a "well-qualified" person, who was fortunate to find work immediately BUT I regret it every day. I earn considerably less than my BA salary (which wasn't huge in the first place!), am micro-managed throughout my working week, have little or no opportunities for promotion or even variety of tasks, and have no opportunities for overtime payments, no fringe benefits, and work more hours a week. The "real world" is not a pleasant place to be in. I would urge those who feel that there should be no changes to take a look outside. You won't like what you see, you certainly wouldn't like what you'd earn and as for terms and conditions, forget it, even with a significant reduction in your salary/terms and conditions, BA will still be a better place to work than most.