Unless otherwise provided in the Standing Orders or the contract of employment, all employees come under one category, that being 'employee'. As such, you cannot have different treatment. Similarly, an employee in the 'staff category' can join a workers' union if they believe their rights will be protected only through that union's representation. If disciplinary action is taken against such an employee, it would clearly indicate that 'workers' are considered part of an underprivileged category, and you do not wish any of your colleagues to be placed in that category.
Regards,
Madhu.T.K