I know that the MI stands for Ministery for the Interior. and I know that 5 deals with home issues/activity and 6 deals with foreign issues/activity. But what, in these cases, does the 'interior' refer to?
Actually, Maggoty. it stands for Military Intelligence (sections 5 and 6).
It stems from the specialities within the original Secret Service Bureau and sections five and six were devoted to Home and Foreign matters respectively.
The phrase is very common in some countries, perhaps if there are a few towns and a lot of fairly inaccessible countryside. It refers to the rest of the country, outside the capital.