They have a right to speak, but that doesn’t give them a right to use someone else’s microphone. Freedom of speech doesn’t automatically give you the soapbox or stage.
Who’s microphone? Oh yes, that one owned and paid for by the British taxpayer. Now tell me again why that isn’t censorship?
Your argument is specious.
The BNP people are British subjects who pay taxes which fund the BBC, so it is just as much their microphone as it is those who oppose them. They have as much right to it as anyone.