....and wouldn't be allowed to express their views in many countries because they are so outrageous...
And that's the exact reason why we need them. The existence of extreme groups in this country, be they racist, religious, animal rights or what ever, ensures the continuation of our enviable, liberal human rights.
Banning groups like that will be the quickest route to a total nanny/police state where we are told what to think. Without extremes it can become difficult to tell what normal and decent should be, or to use the words of the Devil in South Park 'without bad there can be no good, so it must good to be evil some days'
Personally there existence is always proof that something is failing in the mainstream parties as these 'one trick pony' parties only appear when people believe they are not being listened or catered to and want to make their voices heard. They go hand in hand with economic down-turn, job loss or other similar factors.
Just look at the areas where they have a hold, everyone a poor area- low employment, low education, low social mobility. But these are the exact areas that immigrants find themselves being dumped in because of 'middle englands' NIMBY attitude. Add these two factors together and divisive groups like the BNP will then turn the whole thing on it's head and blame the immigrants for the areas problem.
What needs to be done is for the government to spend in these areas rather than appeasing middle england all the time.
As a side note, I am more scared of white middle class, educated labour/tory voters who start a sentence with the words 'I'm not racist but...' . It's the daily mail reader who scares me as they are as bigotted but have power, in comparison the BNP has nothing