I hate quoting television shows, I really do, but I think Stewart Pearson (The Thick of It) really summed it up brilliantly: "Changing this party has been like renovating an old building. You can remove the odd racist beam here, the odd homophobic roof tile there, but at the end of the day you realise that it's been built on a solid foundation of *****."
The problem with the British National Party is that those who support it and those that operate it are nasty, right down to the foundations. You can change the policies and you can change the elite, however, those foundations will remain. Even if you purge the entirety of the party to ensure a clean break in terms of people, the association alone is problematic. A brand so engrained cannot be removed by a simple policy change or re-branding exercise. This particular political party would need demolishing.