As soon as they were invited to participate it was inevitable there would be protests. At that point to cancel the debate would have allowed them to take the moral high ground and claim their right to free speech was being denied. When the debate went ahead, and the inevitable protests took place, it again allowed them to take the moral high ground and claim free speech was being threatened by those opposed to their politics.
I have not seen any reports on the debate so don’t know what actually was debated and how the sides fared. Though, as far as I can tell, nothing much was debated, so we didn’t get the chance to see their views challenged. All I have seen are reports of the protests and claims by Irving and Griffin that the protesters were trying to deny their right to free speech. This has allowed the extreme right to paint their opponents as the true extremists who threaten freedom of speech in this country. It was inevitable this would happen and the Oxford Union, by inviting these bampots to speak in the first place, handed them a propaganda victory on a plate.