The only regulatory restriction seems to be 1.5mm; nevertheless, by that point, it is too late to operate the vehicle. I think it is an implicit guideline that most garages will alert you when the brake pads go below 3mm. In my situation, they would have secured the work anyway, since the garage next is far more economical than the Kia dealer, and, as I said, it would not result in the vehicle being out of service.
I have no objection to the needed task. It is only the situation. If I had been driving even average mileage, it would not have survived until this MOT, and I may be facing rotor damage as well.
In response to your comment on Advisories, if they maintain it at 1.5mm, they will undoubtedly secure the job, since it becomes undriveable.