In the first instance, you should complain to her about this. It seems quite obvious that you think this is unacceptable but any union and ultimately any employment tribunal would want you to try and work it out internally. Also, from a personal point of view, she maybe thinks its just funny. Okay, it's not, but if you don't tell her, how will she know?
If all employees feel the same, you should approach her together (strength in numbers). Ask for a meeting with her and maybe two or three employees representing the whole workforces views. Alternatively, if you don't want to all approach her, write her a letter outlining your concerns signed from the staff.
If its an individual thing, you should still speak to her but will have to do this one on one which people are not always (understandably) entirely comfortable with.
If you have done this already, you have to put the grievance in writing to her formally. After that, you can complain on an official level.
As crusty says, a union can be a good third party. However, if you are a company of under 21 employees, current UK law states that the Company cannot be forced to recognise the union ina collective bargaining capacity. If you are above 21 people, you can have a vote within the workforce to see whether people want to join the union - the unions themselves can give you advise on this. Unions can also give advise individually if you choose to join but cannot represent you on a group level. Of course joining a union is not free and that may deter people from wanting to do so, so maybe you should test the water.
Ultimately, if you've complained to her and put the grievance in writing to her and you feel the behaviour is still unacceptable, you could resign and claim constructive dismissal at a tribunal. You don't need a union to assist you in doing this but they will have more experience of the process. Alterntaively, talk to CAB to see if