Firstly, it it is illegal. The EU Constitutional treaties, aswell as the Convention on Human Rights forbids rejection or refoulement of those in danger in their own countries. Nick Griffin wants to send those who have fled war or threat back to their own countries
Secondly, it is racist. He wants to give grants to British Citizens who are not of British descent to go back to their parents' birth lands. This would include many of my friends whose parents or grandparents are first-generation immigrants.
Thirdly, it is economically unsound. Immigrants are net contributors to NIC, have higher levels of productivity, reduce wage stickiness, clear the labour market and stimulate growth.
They are currently illegal at the moment, because their whites-only membership policy breaches EU anti-discrimation laws. They attempted to reform a few months back, but their reforms were thrown out as still being illegal. But they're still allowed to stand, because the political establishment knows that if they banned them, there would be hell, since its main support base is thugs, football hooligans and neo-nazis.
But there is a very real chance that they may win a seat in the Parliament in a few weeks time. In some areas, they've stirred up racial hatred between the population and the immigrant communities just to win votes.