I will say, in order to have a truly tolerant society, the intolerant must be the only thing not tolerated.
Also, the BNP have been found guilty of having illegitimate recruitment policies (IE, non-white British citizens are barred from joining). Until that is rectified, they should not be considered a political party and so should have no argument that the BBC should give them a fair showing.
On the other hand, free speech is free speech. We can let the racist fascists speak and we can put them down. Hard. The entirety of the British mainstream political spectrum is aligned hard against the BNP - and will unite as one to condemn it. I can see Jack Straw, who I dislike seriously, having a crowning moment of awesome in putting down this detestable demagogue. And they're more than free to do that.
The trouble with Nick Griffin is that he is in many ways an extremely intelligent and charismatic man. He is, to be frank, a fucking snake. He speaks soothingly and addresses peoples' fears who feel they're being ignored by the mainstream, and speaks to the ancient evolutionary instinct of xenophobia. It's clear how he's managed to get his support, and there's a risk that more people will believe his lies here. But, it's the only way to decisively rout the party, and so it must proceed.