I think that GB is partly responsible for the Nick Griffin being elected
Some of those who voted BNP will have been ex-Labour voters, because the BNP target white working class men who traditionally vote labour. However, the main reason why the BNP got so many votes was that traditional labour voters could not bring themselves to vote labour, and so stayed at home rather than voting for another mainstream party. As other posters have noted, the actual number of BNP votes did not increase in this election.
The disillusioned Conservative voters mainly turned to UKIP to register their protest. The Greens did quite well from disillusioned Labour voters, but sadly so did the BNP. Mostly it was voter apathy which did it. If you do not use your vote, this is what you get. I hope those lazy b*stards who stayed at home are feeling proud of themselves.