Right to be accompanied
A. You have a right to be accompanied to a disciplinary hearing under section 10 of the Employment Relations Act 1999. You can bring a claim against your employer in an employment tribunal for a breach of this section.
The claim must be issued within three months, and the tribunal can award up to two weeks’ pay.
Whether a meeting counts as a disciplinary meeting or not depends on what happens — or what could happen — at the meeting, and not what the employer decides to call it.
Since you were given a warning at the meeting, it’s likely this would be viewed as a disciplinary hearing, and that your employer’s refusal to allow you a rep was a breach of your statutory right to be accompanied.