I'm very much against UKIP and just about everything they stand for, but this isn't a good argument against PR. If people are going to vote for UKIP, even if it's against their best interests, then UKIP deserves to get represented. The flip side is that there are plenty of left-of-Labour parties who would also do well under PR, so I don't think the system would really swing Parliament rightward. I guess you could argue that from a leftwing perspective a UKIP-Tory coalition would be worse than a pure Tory administration, but it's splitting hairs - there are plenty of people on the Tory right who are essentially UKIPers who just don't want to join a small party.
If anything it's faintly admirable to see Labour supporters supporting PR, because PR is not really going to help the Labour party. If the last election had been run under some kind of PR system, Labour would still have lost.