The Midford family consists of nobles of a high hereditary rank, and they also serve under the Queen. Their primary duty is to defend Britain, hence the head of the household, that is the Marquis, is given the title of the Head Knight.
The distinction between a Marquis and an Earl is that the former's land, called a march, is on the border of the country, while an Earl's land, called a county, often is not. Therefore, a Marquis is trusted to defend and fortify against potentially hostile neighbors and is thus of greater significance and ranked higher than an Earl.
Additionally, it is crucial for the members of the Midford family line to be adept with swordsmanship or fencing.