Ballots are counted by independentt organizations. There are a number of companies that provide such a service, they are also used in party leadership elections, for local referdums etc etc.
Geneneral election ballots also have a serial on them, and can be traced to an individual if a court demands it. That doesn't stop people voting.
This is getting silly. Some are against the strike, some are for it. Making up stuff such as the ballot is not private or there are 20 747's at Cardiff does no side any good.