The Royal Observatory in Greenwich, UK is the location where the prime meridian (0° longitude) was established in the Nineteenth Century, thus providing for a baseline of geographic standardization. You can follow that line (or any other line of longitude) up to the North Pole.
But the North Pole is usually not magnetic north, which is where compasses will point. Magnetic north does, however, move. And, and the moment, it allows compasses in Greenwich to point to both true north and magnetic north. The British Geological Society reports:
The angle a compass needle makes between true north and magnetic north is called declination. As the magnetic field changes all the time, so does declination at any given location. For the past few hundred years in the UK, declination has been negative, meaning that all compass needles have pointed west of true north.
The line of zero declination, called the agonic, is moving westward at a present rate of around 20 km per year. By September 2019, for the first time since around 1660, the compass needle will point directly to true north at Greenwich, London, before slowly turning eastwards.
-via Kurt Schlichter | Photo: Virginia State Parks