I would bet money on the fact that so far they have had a low return of the ballots. I have flown with quite a few people on my last few trips who have not yet voted. Most have only been here a couple of years and are totally confused by the whole shebang.
Some even think that because they belong to the Union, they have to vote yes, so to clarify here are the facts:
The vote is your personal choice YES or NO.
The Union can do nothing to you if you vote NO.
If you do not vote at all, you might as well have voted yes, because it is only returned votes that count.
If you vote Yes then you are in breach of your contract, and the company could issue you with a different contract before allowing you back to work.
You can vote no, and come to work on strike days. It is your right under UK employment law, and the law protects you.