@Antigon - 'Disgusting, vile, sickening, but not a crime' seems to be the new 'I'm not a racist, but...'
That's a rather odd statement. Just because something is disgusting, vile and sickening doesn't mean it's a crime. In order for it to be a crime, a law must be broken.
You may feel it should be a crime but, again, that doesn't make it a crime, unless a law has been broken.
Given no one ever gets prosecuted in Northern Ireland when effigies of the Pope or Martin McGuinness or similar are burnt, I suspect that burning an effigy, regardless of whether the motivation is racism, religious bigotry or hatred of people in social housing, is not a crime. The fires in NI take place on public land, rather than private gardens. If that is not a crime then doing the same in a private garden in very unlikely to be a crime.
I think the people who did this are absolute scum. I think they deserve to be ostracised and reviled for their actions. However, unless they have broken a law that is on the statute books, they cannot be prosecuted. Perhaps burning of effigies should be outlawed but I would be concerned about where that would lead in terms of freedom of speech.