A degree in politics isn't totally worthless. When compared to other arts degrees such as sociology or English lit, politics has the advantage because the degree requires some knowledge of public admin, economics, law, the media, and almost anything else depending on where you go to learn it. Most unis have their own unique way of teaching this degree.
In terms of career prospects, your obvious choices would either be working for political parties or the civil service. However, unless your from a Russel Group uni this could be hard to get into. The people I know who've finished this course mostly start off working for MPs just for experience, then move up to working for wherever they want to.