]Can anybody tell me how high an ensign's rank was? ... I don't think P&P ever mentions his rank in the militia...

militia regiment: For most of Austen's adult life (1793-1815), England was at war with post-revolutionary France. The militia were a mobile military force, established in response to the fear of invasion, that moved from place to place, largely in the south of England. They were thus distinct from the 'regulars' who had fixed camps throughout the country. This new practice of extensive billeting of troops came in for considerable criticism.
ensign: An ensign was the lowest grade of commissioned officer in an infantry regiment. [P&P notes]