Right to be accompanied
A. The right to be accompanied by a union rep or workplace companion comes from section 10 of the Employment Relations Act 1999 and applies to disciplinary or grievance hearings.
Section 4 of the Act defines a disciplinary hearing as one that could result in a formal warning; the taking of some other action; or the confirmation of a warning or some other action taken.
This means your legal entitlement to a rep depends on the possible outcome of the meeting. If it could result in a disciplinary warning (even an informal one if this could be relied upon in the future), you would be entitled to bring a rep. If HR is saying that the meeting is informal, you would be able to challenge any attempt to impose a warning.
If you have a disability, then permitting you to be accompanied by a union rep could be a reasonable adjustment under the Equality Act 2010 and you can make a request on this basis.