The problem for UKIP is that there's no real will on the right to replace the Conservatives with UKIP MPs. (I think they have a better chance regarding MEPs.)

Without some defections, any constituency that looks like tipping to UKIP will see huge investment from Tory funders to make sure that the Tory wins...

As such, it's very hard for UKIP to gain MPs as they have little support in non-Tory areas.