Adloyada has the details. As she points out, the BBC interviewer didn't even bother to call Irving on any of his remarks, she just let him ramble on.
But what if she had? One of the more idiotic modern-day tropes that has been promulgated in part by the media is that there are two sides to every story. Not so. The Holocaust is a fact--not an opinion. And giving Holocaust deniers a respectful hearing only encourages them.
I remember once watching the Sally Jesse Raphael show (remember her?) that tackled Holocaust denial. They had some dirtbag up there expressing his views and then they drag out this poor old woman with a tattoo on her wrist who was sobbing while she told her story. I remember Sally going up and giving her a hug. It was appalling. Putting a Holocaust survivor on display as though she were an artifact or museum display was hideously exploitative. And allowing some shithead to natter on about his poisonous views on national television is inexcusable.