I agree, but the video at the bottom helped.

You can draw a line between your location and the north pole, they talk about three variants:

- Magnetic North: Shortest surface line to the magnetic north pole (simply in the direction of the compass at your location).

- True North: Shortest surface line from where you are to the geographic north pole (based on the rotation axis?).

- Grid North: A line to the same geographic north pole, but aligned to the longitude lines (EDIT: for a local UK grid standard, slightly different from the global one). I didn't fully understand the subtleties of why it's different from True North, something about the projection. Not sure if it's exactly to the same north pole, the rotation axis might also change slightly and I assume that the grid north point is fixed by convention?

They are saying that there's a particular point where all three lines point in the same direction, and that point is moving.

This video explains it a lot better: https://www.youtube.com/watch?v=HcFvegnQpPo

Basically, there's a "true north" which is where the axis of rotation of the earth intersects with its surface, and for local mapping in the UK there is a grid which is not longitude and latitude but instead locally flat (i.e. each grid square has the same area) but that means that it only lines up with true north along one line that goes north-south through roughly the middle of the UK.

Then the third is magnetic north, and this is one that both varies from place to place and with time. Magnetic north, as in the place where the earth's magnetic field intersects with its surface, doesn't necessarily line up with true north. Even more complicatedly, compasses don't actually point to the magnetic north pole, but can be about ten degrees off to the east or west depending on where you are on the planet's surface (for most of the surface. Near the poles it can be wildly off. Obviously if you're walking around near magnetic pole your compass is going to be all over the place). And to top it all off, this changes from year to year. If you have charts for navigating by compass, it will give you a table and formula for correcting what you read on your compass to grid or true north, and those depend on the date. This also needs to be kept up to date with an almanac or similar because it can't be predicted arbitrarily far into the future.

What this means, is that there's basically a funny-shaped line on the earth's surface where the magnetic north happens to line up with the true north, and this line moves over time (you can see a current map here: https://www.ncei.noaa.gov/sites/default/files/inline-images/...). At the moment, this line intersects with the line where the UK's grid north and true north line up within the UK mainland, but it's been moving for the past three years and this intersection point will soon be off the coast.

If the shortest route from your current location to the English channel crosses the M4 then you're in the north, otherwise you're not.

I think they are referring to a moving point on earth's surface where all 3 norths appear to be in the same direction. I agree it wasn't clear.

What's also unclear to me is how all 3 could reliably be colinear, but maybe it's an aspect of spherical geometry that eludes me.