Obviously what you describe here is an impossibility, with some of your requirements contradicting others. But is it really a necessary requirement that BNP-like parties get no or very few seats? History in continental Europe shows that such parties get less extreme as they get represented in parliament and accountable to their voters, even when they never make it into a government (I oppose the Belgian cordon sanitaire in principle, but wouldn't want to form a coalition with the Vlaams Belang, either). And I can see why you'd have issues with coalition governments, but in most cases, having a government platform that makes compromises between the stances of several of the most popular parties who together have a majority of the popular vote is likely to represent the stances of more voters than having a single party government that represents less than 40% of the popular vote.