His speech IS incitement to harm others NOW. He is essentially a propoganda officer for violent neo-nazis. The entire philosophy his revisonism supports ends in violence. There is no 'Shiny Happy National Socialism' movement; Irving is directly associated with people who support the use of terror in Germany to destabilize the given social and political system to pave the way to a new NS revolution.
This ignores the responsibility of the acting agent. I can say all day that I hate Jews or blacks or white folks from Michigan, but I'm not killing them. If some wanker decides he likes what I say and decides to kill someone, the killer is responsible for the murder. Placing blame on what someone says and not on the actual murderer's act is the same logic that the Nazis themseleves used.
"Hitler ordered me to do it! It wasn't my fault! I was only following orders!" No, it doesn't work that way. We all have ultimate responsibility for our actions.
Saying something negative about a person doesn't harm that person. Basic human rights don't include the right not to be offended. Once we squelch speech, we only make it more attractive to engage in. I never considered burning a flag, but if it was illegal here, I would promptly burn one, just because it is a violation of my natural rights to restrict my speech where no one is harmed by it.