From as much as I've heard about this party, I still really don't see why the BBC would actually offer him a soapbox to stand on. I understand the whole impartiality thing, but can you honestly say that the entire ideology of the BNP is mainstream enough to warrant attention?
This really only benefits them as a way of getting the attention that they cannot get regularly. I'm sure their supporters will now view this as a personal attack and actually try to empower their base. Hell; even someone who may not agree with the majority of things they say might catch light of one point he makes and actually be drawn to that. Believe me, that is possible; just look at all the lower-class people here in the US who believe the trickle-down theory benefits them, or angry right-to-lifers who are indifferent to war or the death penalty.