I doubt they'd have a case for slander, since all the council said was that they thought Ukip - not the couple themselves - were racist, and they seem to have said it only to the couple, not to others. It's the couple who've chosen to publish the allegation more widely, and "publication" to others is needed in cases of slander.
If I tell people you'e a fool, I may be slandering you. If I just tell
you you're a fool, I have no case to answer.
As to the truth of the matter, who knows? Ukip does seem to draw its support from the same sort of people as the BNP
http://www.searchlightmagazine.com/archive/ukip-at-the-crossroads
So the council may have grounds for worrying about the couple's attitude to the children they foster; and their duty is to the children first, not to the foster parents. The story doesn't say where the children come from, but I would have thought, on the basis of the limited facts in the Telegraph, that the council at least have cause for concern.