the Countess was saying that the first mention of the song was in 1850

I said that was the first mention of the actual horse fair held at Widecombe, not the song. As Jon (post and link above) points out, the notes on the folk.info site, which appears to be currently down, versions of the song have been collected which refer to other fairs and many of the texts were undoubtedly earlier than the most widespread 'Uncle Tom C' one which Mr Knightley draws on.