Between 5-7% of Labour voters in the 2010 election are now voting UKIP, compared to 14-18% of Conservative voters in the 2010 election - they are not even remotely closely impacted by UKIP.
Of Liberal Democrat voters - 32% are now voting Labour compared to 15% Conservative.
If it wasn't for SNP making huge gains we would be looking at a clear Labour majority this election due to the collapse of the Liberal Democrats & rise of UKIP. The increase in popularity of the Green party has also minor negatively impacted the Labour vote (but not likely enough to change any results in seats)
http://may2015.com/category/drilldown/
Finally, looking at the recent polls 6% of 2010 Conservative voters have switched to Labour compared to 3% of Labour voters the other way around - these are likely to be in swing seats which will have an additional impact.