***** NOT A TEACHER *****
While you and I wait for the answer, I found something in my favorite spot: the "books" section of Google. I typed "northern London" (with the quotation marks) and found quite a few examples.
Here are just four:
1. "Sigmund Freud lived, worked, and died in this spacious three-story house in northern London." -- FROMMER'S LONDON (2011).
2. "[I]t was resolved to erect on this spot a place of popular entertainment for the working class of northern London." -- OLD AND NEW LONDON (perhaps 1878).
3. "There is a good view of northern London and the country towards Highgate, Hornsey, and Essex." -- BRADSHAW'S HANDBOOK TO LONDON (2012).
4. "A venture to northern London takes in Regent's Park and its beautiful rose garden." -- NATIONAL GEOGRAPHIC TRAVELER (2011)
Student or Learner