Agree with or don't agree with his policies they DO genuinely challenge the status quo. He is much further left and that is a genuine threat to, for example, the very well heeled owners of most media here. For all his rabble rousing, outsider persona Nigel Farage is much more a business as usual candidate if you like your politicians posh, your taxes low and your wealth tax non existant. So it makes more sense to signal boost Farage (even if in a disapproving way).
Besides the Greens would be judged more for issuing outlandish statements or making themselves ridiculous for attention. See Ed Davey. Whereas no publicity is bad publicity for Farage. It's not fair but it is what it is.