They have representation in other ways. For instance in Britain, where it is illegal for police to unionise, there is instead a government-funded Police Federation, which independently advocates for police working conditions and supplies advice and representation for police in disciplinary affairs. The main difference from a union is that it is free to members and that police are not allowed to go on strike. This removes a lot of the worst abuses of power.
They have representation in other ways. For instance in Britain, where it is illegal for police to unionise, there is instead a government-funded Police Federation, which independently advocates for police working conditions and supplies advice and representation for police in disciplinary affairs. The main difference from a union is that it is free to members and that police are not allowed to go on strike. This removes a lot of the worst abuses of power.