Should the BBC be inviting BNP leaders on to a discussion programme like Question Time? If they do invite them, should Labour politicians agree to share a platform with them? Yes and yes. The first yes is on account of the BBC being a public service broadcaster with an obligation to treat legal political parties 'with due impartiality'. Note, therefore, that the opening question here is different from the question of whether, say, a liberal newspaper should give space in its pages for racists or fascists to present their ideas. The answer to that question is, in my view, no. As a general rule anyway, a paper like the Guardian should not host writers from the BNP for the same reason that it shouldn't host - though it sometimes does - leaders of organizations with an overtly anti-Semitic programme and ideology. This follows from the paper's aspiration as liberal. It shouldn't aid those who propagate poisonously anti-liberal ideas. The BBC operates according to different public obligations and constraints.
If the BNP go on Question Time, then so should Labour spokespersons, to challenge and refute what the BNP have to say. In some circumstances a better way of denying the moral legitimacy of a viewpoint may be to ignore it. But once it's being aired on the biggest broadcasting operation in the land, to do that would be folly. Better to make it the target of careful, but also forthright and contemptuous, rebuttal.