For the people who have raised the very real fear that BNP and UKIP voters' votes will spill over into the results of the mainstream parties:
This is a good point, and it could happen. This is why the mainstream parties have got to stop being so damned lazy about stamping on disgusting and divisive ideas and lies propagated by "parties" beyond the pale.
Labour and Conservative have been disgracefully crap about this, because they're not at all afraid. They're not really fighting for our votes, nor for the society they believe in. If we change the current system, they will have to do better, or they will lose ground, in some cases seriously (as Blatherskite fears).
I know it sounds arrogant, but in a way, I'm not even afraid of a BNP candidate's getting in Parliament. What the hell is s/he going to do there? S/he will be marginalised, and will give the others enough of a shock to make them get off their arses and be cleverer and more humane about creating a society in which no-one wants to vote for a hate-mongering racist. Okay, the dignity of Parliament will be dented by offering "legitimacy" to such political views. However, again: those who care about the honour of Parliament can make that right.
Sinn Fein, the political arm of the IRA, has had candidates elected to Parliament (although I'm not sure they take up their seats), and there is power-sharing in Northern Ireland. Although some people would never agree to even look at Sinn Fein, thanks to past history, UK/Irish politics have survived the introduction of Sinn Fein and powersharing.
That's not to say I believe the IRA/Sinn Fein and BNP have the same potential future. The IRA used much bloodier and more bullying means than I am aware the BNP have. However, their politics are of decolonialisation/fair representation/anti-discrimination, which can be sought politically, rather than bloodily, and thank God they are doing that now. Meanwhile, the BNP, even if stupid and not as bloody, have a much less defensible politics of discrimination and disintegration - which is why I don't believe they have a political future, except when other political parties aren't doing their job to make this a decent society.