Well as far as I can make out, the Lib-Dems are prepared to go into coalition with either Labour or the Conservatives; the two mainstream parties havent said but probably will reciprocate. SNP will go into coalition with Labour but Labour has ruled out a coalition with SNP. I dont think anyone would go into coalition with UKIP apart from the Tories. I know (being a member myself) that there is a great deal of internal opposition within UKIP to go into coalition with the Conservatives. Farage has always ruled it out. If he does enter into a coalition I think a considerable proportion of the membership will tear up their cards.
Issue by issue arrangements are certainly possible, but a UKIP Conservative tie in is, I think, unlikely.
> Farage has always ruled it out.
As noted he’s stating he’s willing to lead UKIP into a coalition with the Tories if there’s a referendum on the EU.
Farage is in a real spot, as you said, there are members who say they will dump UKIP if they join a coalition. If UKIP isn’t in the next gov’t, it will fade out like many an insurgent party has in the past.