Three preconditions for the rise of UKIP:
1. Economic problems, with contraction in wages for those in the bottom half of society.
2. Year-on-year rises in net immigration.
3. A Tory government.
1. is reflected in the demographic - average UKIP voters are below average income. They are struggling, and hit hard by cuts.
2. This is their trump card. They are the only party that can honesty say they can do something about it. They also link 2. to 1.
3. Under a Labour govt, the tories can, and did, take the UKIP position. UKIP pointed out at the last election that the tories would not be able to live up to the limits they set. UKIP was right. They couldn't. In opposition the tories could, and did, position themselves as the anti-immigration party. In power, they cannot - immigration continues to rise under their watch. UKIP can now very credibly take this space, and point very credibly at how they pointed out at the last election that this would happen.
I would add a 4. Nigel Farage. If Farage had been killed in that helicopter crash, UKIP would not be the force they are today. Under the likes of Neil Hamilton, they would not be winning by-elections. The necessary preconditions need to be there for him to exploit them, but he has had the skill to exploit them. He's probably the most impressive mainstream politician in Britain today.