I've read their policies and can't find anything vaguely racist
I don't think they are a racist party in the same way as the BNP. The BNP has racist policies. UKIP doesn't but some of their candidates have expressed racist views.
UKIP were founded to get the UK out of the EU which is perfectly legitimate. However, if you campaign to leave the EU and end EU free movement you will attract both legitimate supporters and those with more toxic motives. They have not been very good at policing themselves with the result that some of their candidates have gone seriously off message with comments that are racist, homophobic, misogynist, etc.
Is Nick Griffin racist? Clearly yes. Is Nigel Farage racist? I don't think so.
There would be no point campaigning to get Griffin to expel all racists from the BNP. There would be no-one left. There certainly is a point in challenging Farage to kick racists out of UKIP.
I do not support UKIP. I would never vote for UKIP even if they were the only party on the ballot paper. I think it is a party that contains racists but I do not currently think it can really be called a racist party. Whether or not it becomes a racist party depends on how willing Farage is to fulfil his promise to clean up the party's act.