Comment by rcxdude

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.