I think what is most concerning regarding the growth in electoral support for the BNP is that it's coming at a time when Britain is enjoying a decade of economic growth and general prosperity.
Support for the far-Right usually grows when times are hard and unemployment is rife (The high percentage that Le Pen gained in the French Presidential elections while the country suffered from high unemployment is a recent example) so a rather large question mark hangs over the support for the BNP.
It's possible that people have simply become more racist, or that they simply don't care anymore (In much the same way that being branded an Anti-Semite is losing it's authority). It's possible too that concerns over the largest influx of immigrants in history is genuinely causing friction and tensions, or it's possible that the tabloid attacks on immigrants are influencing the way people vote.
Either way, it's a (Slight) cause for concern that during a period when the economy has been stable and beneficial to most, many former Labour voters have felt the need to vote for the BNP.