It's just one bunch of fascists attacking another...
Whereas I have no time for the BNP, I find this kind of suppression of free speech deplorable. Can you imagine if we ended up with a government staffed with people of the mindset of those who perpetrated this attack? What would they do to anyone with the temerity to publicly disagree with them? In reality they're just another brand of fascist.
Many would regard a diversity of political opinion, therefore political parties, as a sign of a healthy democracy, even if some of those opinions are rather unpleasant.
The sad fact is that there are people in our society who do agree with the policies of fringe movements such as the BNP and the Greens, both of which seem long on coercion and short on consensus, and who should be allowed to vote for whichever party they believe will best represent their views.
I object to those who have so little regard for their fellow man that they assume that we are all so stupid and ignorant of history that we have to be guided (by them, of course) for fear that we can't see how moribund most extremist philosophies are, and therefore choose not to vote for them all by ourselves.
If they were to make more of the surprisingly large number of criminal convictions against some members of the BNP's inner circle (for offences such as gang rape, armed robbery, and violent assault, rather than "political" crimes), or point out the lamentable attendance record of many of their elected councillors, demonstrating that representation of the people is not a priority for them, then they would arguably be far more effective.
Assuming this attack did happen, it seems far more likely to reinforce the beliefs of those who already support them, and get the sympathy (!) vote of those who are wavering, than to have any significant negative effect on them.