Farage didn't actually say that UKIP was open to forming a coalition with Labour or anybody following the election in 2015 but the papers ran with the story anyway. What he said was that following the election, "UKIP may well be needed in a coalition" and said that there is no way he could work with David Cameron.
The views of the membership on a coalition have been made pretty clear over the past year or so as the prospect of UKIP gaining representation in the House of Commons has come tantalisingly close. The Lib Dems have shown what happens when you abandon your principals for a sniff at power and there is no desire amongst the membership or the leadership of the party to do the same. UKIP MPs may support a minority government in exchange for some key concessions but a coalition would be political suicide.
Peter Mandelson, who isn't allowed to criticise the EU otherwise he'll lose his very generous pension, said:
Beginning 2013 by placing this large and indefinite question mark over our membership of the EU, and all the trade and investment privileges it brings us, can only be described as economically insane
With people like the disgraced communist Peter Mandelson - and there are many, many disgraceful communists in the Labour Party - they are no more likely to hold an in/out referendum on membership of the EU than the Tories are and any mention of electoral deals in exchange for one is just Farage spreading dissent in their ranks.