They'll affect the marginals but won't get the seats to represent their popularity. They are by and large right wing and ironically the right wing turned their nose up to the suggestion of a form of proportional representation. PR would get them more seats.
I think people vote UKIP as a protest against Brussels pigs in the trough and lax immigration policies which work against the british working and lower middle classes. Skin colour is of no relevance whatsoever btw for the vast majority. Many prospective UKIP voters would be wary of them actually...
UKIP will do better than 2010, Lib Dems will do worse but not as bad as people say, Labour will do a little better but Milliband might be a liability, the Tories will do a little worse. A bit like 1992 but with UKIP thrown in the mix and the government being a coalition rather than Tory with a...