I think that they should be allowed to go on, they are a legitimate political party and they should be heard. Like others have said I think that silencing them gives them a certain underdog appeal. I'm also a firm believer in free speech, I do not agree with what they say but I support their right to say it.
Personally my opinion is that they really don't want to go on it. Nick Griffin is an idiot and their arguments don't really hold any water what so ever once exposed to the light of questioning.
Their best hope is that they get banned from appearing, that way they get lots of publicity and don't actually have to face up to some tough questions.
Now that it appears that they're not going to be banned they seem to be insulting the other guests in the hope that that will get them banned.
If that fails I wouldn't be surprised to see them try and egg on some anti-fascist backlash that either disrupts the show or prevents them for appearing for security reasons.
They can then play the "help I'm being oppressed card" and avoid the embarrassing and revealing questions.
For the record I'd also like to make clear that I believe the BNP to be a bunch of f**k headed numpties whose nastiness and spite are in inverse proportion to their intelligence.
That was quite strong.
But my Grandparent's generation died fighting their sort, I'm ashamed that certain people forget that so quickly.