That is just a myth. Yes the protest vote is important, but thats far from the only reason. I've had voters express anti-immigrant/racist sentiments, occasionaly mixed with a kind of old labourish attitude they play to. Their old tory and then some attitude to crime has also come up. All expressing a positive desire for the BNP to get elected. All that from rather limited personal experiences.

It might be nice to pretend its all those bad main parties faults- and to a large degree it is. They win seats largely in what are basically labour one party state areas. But in these areas they could probably still get into teens %-wise without such help.