The Department is concerned with the occult defence of the Realm and the Realm in question is Great Britain. The Department has a fairly loose definition of what constitutes Great Britain. After all a threat to any part of the planet Earth could eventually become a threat to Britain.
The Department has been around longer than you think. The Department was first called the Department during the second world war and has been funded by the British Government since October 1st 1939. That’s how the official history of the Department has it, anyway. However it seems to be impossible to find a part of British History which is untouched by the Department in some form.