As a 'worker' the law says that you have a legal right to be accompanied in disciplinary hearings by a trade union official or a fellow worker of your choice. If you choose a union official this may be a person from any trade union, whether your employer recognises that union or not, who is certified by that union as having experience in dealing with disciplinary cases. (Where you are a member of a union recognised by your employer, the person you may choose to accompany you is likely to be your current union representative.)
If your employer has a disciplinary procedure it will normally have a clause on representation and it might provide for you to be accompanied by individuals other than those highlighted by your legal right, for example by a solicitor. If this is the case then your choice of person to accompany you will be wider than the statutory provision. Also, where the consequences for your career of a disciplinary charge being upheld are severe (for example, this is your only opportunity to avoid being barred from practising your profession), you may be entitled to hire a lawyer to act as your representative.