I think the UKIP are seen as a protest vote by a lot of people (just to qualify, I wouldn't vote at all at the moment as I don't believe in any of the parties or anything they say).
I think there is real, deep felt anger at the way the main parties (Lab, Lib Dems, Cons) seem to look down at the voters, say one thing in their manifesto but do another and just generally have an arrogance about them that originates from a political class that is currently bred in Oxbridge and has very little experience of the real world or real worries. Let's face it, how many current politicians will have to worry about housing or their childrens education or a pension - very few. I also think that politicians these days seem to be very much career politicians and will cosy up to business or unions who will help them further their career once they have left office. Very few seem genuinely concerned in doing what is right for the country but rather make policies (or change policies) to ensure they are re-elected. They all seem very cynical, immoral, grabbing and untrustworthy.
UKIP appeals because people know what it stands for (even if it does ride on one main policy) as opposed to the Tories or Labour who don't seem to stand for anything at the moment and have become red-Tories, blue-Labour with policies that many people don't really worry too much about such as gay marriage or windfarms.
People mostly vote UKIP because people don't like being told not to do something and people hate the whole "you're racist if you oppose unlimited immigration" because they see for themselves that it is not a racism issue but rather an issue of overcrowding, pressure on public services, school places, the NHS which the big parties are currently putting their heads in the sand about for some reason, when the reality is very dire for a lot of people.