Had a think about this this morning... here are my musings.
For all their hot air and bluster present in the lack of a true political manifesto, the mainly ill informed electorate of the UK appear to still consider UKIP a one-issue party - that issue being to pull out of the EU, linked with their potentially xenophobic stance on immigration ("They dun took our jobs wot I didnt take cos I was happy on them there benefits!"). People are not voting for them for their policies on climate change, education or anything else. As a result, in elections that are linked to that issue (like the recent Euro elections), especially ones with low turn out, their level of support will inflate, especially by absorbing more Euro sceptic Tory voters. Come general election time, UKIP have shown themselves over the last 21 years to be out of their depth due to the broadness of the issues at hand, and their support stumbles - no doubt a hard core of supporters remain, but other voters will vote Tory/Labour/whatever based on wider issues when they may have voted for UKIP on a narrower platform. The obvious sitting duck of a target for the other parties in the general election is to focus on wider issues like the NHS, education, defence etc. where the larger or older parties actually have concrete policies that appeal to the majority of the electorate, and to avoid UKIP the one and only platform that gains them "popular" (and I use that word in a loose sense) support in narrow issue low turn out elections. Press Farage on their plans to build 3 aircraft carriers, or a payment based NHS, and avoid him bringing up the same old "lowest common denominator" drivel that panders to fairly ill informed fears. Hoist them on their own petard, to coin a phrase, instead of bashing them in to bogeymen who can play the victim - which suits them as they can pull the whole "the UK is a victim" thing. Labour has the most to win, clearly, by UKIP standing to slip Tory voters. Ironically, UKIP being on the right could force Labour back to the left, leaving the Tories in the centre.