I think the reason the term, 'white trash', might not be considered racist in the USA is because it is a term used by all ethnic groups, including whites, to denote poor, ill educated white people. He might have been better advised to use the alternative and less racially motivated term, 'trailer trash', but he may have considered this inappropriate in Britain, as we do not use the term 'trailer' to denote 'dwellers in caravan parks'. (Yes, I doubt it too, but he seems a genuinely pleasant guy, so I'm prepared to give him the benefit of the doubt on this one occasion.)