A much better way to combat the BNP is to look at the facts.
Of course, there are always a hardcore of nasty white supremacists who think Britain should be for whites only, but I'd wager a much much much larger proportion of the people who vote BNP are much less devoted to such a puritannical nasty ideal, and are more concerned about the guff you read in the tabloids about immigrants getting 1000 quid a week in benefits and so on.
These are the people who need persuading the other way.
Show them how hard it is to actually get into Britain. Show them that even legitimate immigrants have 'NO RECOURSE TO PUBLIC FUNDS' stamped in big letters on their visas, and as such have no chance of getting any kind of benefit. I've got a Chinese friend living here who did his (Architecture) Degree at Newcastle Uni, now earns about 35k a year on which he pays full tax and NI, and yet he can't even get child benefit for his child, or tax credits or anything. So in reality, immigrants such as him are paying the same tax as everyone else, and getting fewer benefits.
If I go on JSA in the UK, because my wife is no recourse, I can only get a single person's JSA and single person's housing benefit allowance (which wouldn't cover the whole rent).
These are the things people need to be told about.
I don't believe all the people who voted BNP are 'non whites out' types, they're probably more low-level Sun-reading gripers who wanted to protest.