The world is a very different place to when the Nazis came to power. Notably the absence of world wars, causing many other factors that gave them the rare opportunity to rise to power.
Today, the BNP would never become a ruling party. If something incredibly unexpected happened that allowed them to (e.g. extra terrestrial intervention, brain-washing drugs in the water supply etc.) then a war would most probably ensue, and I would leave the country, or stay and fight for what I believe in if I had the guts to that. I'd like to think I do, but when you're looking death in the face, who knows what you're really capable of.
Edit: I would share similar views for nationalist partys such as the Scotish Nationalist Party and Plaid Cymru, as it is my opinion that no one should, or in fact will, get anywhere, by offering a better deal to particular geographical areas, unless they are genuinely in need of it, but even then there are limits.
As for the Greens, I can imagine a day when they rise to be at least the second largest party, but there will always be ignorant people who would rather stay as they were. Main stream parties will eventually copy green policies and scale them to a more amicable extent and drain The Greens of many votes.
Lib dems, well who knows. That's another story for another thread really. I would have thought that before Ming 'modernised' them (read: moved them to the right, or if you share my opinion 'ruined'
) they would have been more successful, but it seems the destiny of UK politics is a set of 3 main parties covering an area of the political spectrum no larger than a pin head. Then you ask yourself: is it really a democracy anymore, or more a one party state?
But hey, I've gone off topic.
) - but as such its important that their voice be able to be heard 
