I'm no political pundit, but...
My medium-term predictions for UKIP:
By 2020, despite electoral success (or failure, which by my calculations is less than 13 MPs - you know, like 2% of Parliament) in the 2015 general election, UKIP will be overrun by Red Kippers (Left leaning, pretending to be right leaning; so herrings really, not kippers) and nimbys who haven't travelled more than 20 miles from home. Blue will drain from the purple in less than the course of a single parliament - regardless of who wins in 2015. UKIP's fatal flaw is that libertarianism can't work alongside protectionism and that evolution ALWAYS wins. You either bend, or you break. Or shatter.
Despite many Kippers getting off on the idea; UKIP won't hold the balance of power in 2015. Not because they're right about the EU, or the belief that they'll be forced to marry someone of their own gender, or the fact they're cheesed off that people with no connections, no contacts, no family or community support structures can get a job in the UK, based purely on their work ethic, but because they don't have a coherent narrative, a poor grasp of basic mathematics and economics and that most people prefer the devil you know than the one you don't. And that ANY increase in the UK voter base in the future will be politically astute, internationally-minded, modernistic, young people. Everything UKIP ain't.
Much of the noise UKIP makes is part blue, and now overwhelmingly, part red (so THAT'S how they get purple), and whichever way you look at it, it reeks of populism, high on emotion - like the YES campaign in Scotland - which doesn't translate very well ballotboxwise. That David Cameron won't be around forever will not hurt the Tories in any significant way; I couldn't say the same for Nigel Farage and UKIP. I doubt anyone can.
In countries with fairly homogenous populations - such as much of the west - apart from minor blips lasting no longer than a decade or so, politics will ALWAYS settle into a binary, two party system - one FOR high taxation and spending, and one AGAINST. It's a lot like two rowboats side by side and you have one foot in both. And then, as one boat moves ahead of the other, you make a choice. Jump on one of them, or lose a pelvis.
Bottomline? Post 2015, UKIP will turn into a Nationalist-Socialist Party, hurting disaffected Labour voters more than any other party.
My long-term predictions for UKIP:
There won't be a long term.