Nature abhors a vacuum...

Rightly or wrongly, there are plenty of people who for various reasons are concerned about immigration and multi-culturalism. The main political parties do not seem to want to talk about these things, and so these people gravitate to the BNP - not necessarily because they agree with their policies, but simply because they are the only party willing to talk about the things which are relevant to their everyday lives. So if you want to defeat or marginalise the BNP, the answer's simple, isn't it?