Yes. You have the right to be accompanied to any meeting that is part of a formal process that could result in a disciplinary sanction. This could include, for example, a formal performance warning, loss of pay, a transfer, demotion or eventually dismissal.
Only in the case of an 'informal' 'quiet word' with your line manager do you not have the right to be accompanied (although it is still worth asking, if you would prefer to have someone with you). This kind of informal meeting must never be taken into account by your employer when deciding whether you have already been warned about the need to improve your performance before deciding to dismiss.
You have the legal right to be accompanied by a trade union rep of your choice or a work colleague.
The trade union rep must have appropriate experience or training to act as a companion and should be certified in writing by the union.
Check your employer’s written procedure in case a right to be accompanied is offered that is wider than the statutory minimum.
If you have particular needs, for example because you have a disability or language difficulties, your employer must take these issues into account and make any necessary adjustments to the hearing arrangements – including your right to be accompanied – to eliminate any disadvantage. Otherwise your employer risks breaching the Equality Act, as well as carrying out an unfair dismissal if you are eventually dismissed.