I make no apology for copying this from my reply to AOG's recent post, on almost the same subject ::
According to electoralcalculus, south of the Border, the Tories stand to lose more votes, although not necessarily many seats to UKIP, than Labour. But with our first-past-the-post system, UKIP may not end up with many seats at all. They may come third or even second in a lot of constituencies, but coming first is the only thing that matters in General Election.
But it would seem, from the same source, that the SNP is likely to gain about the same number seats from Labour, as it will from the LibDems...about 10 from each. But lets not over-egg the SNP pudding...the YES vote only managed about 38% of the available votes that could be cast, so a largish minority. Not insignificant but hardly a king-maker position either.
Overall, my best estimate is that UKIP will just make a Labour victory more certain than not, as Labour is still consistently ahead of the Tories in the Polls ::::
http://yougov.co.uk/news/categories/politics/