Map data in a national grid

The same files on two maps. Left: Web Mercator, with the grid data converted as it loads. Right: the map drawn in British National Grid (crs="EPSG:27700"), satellite imagery warped into it. Pan either one; the other follows. Hover a town for its grid reference.

Web Mercator
Grid data converted to longitude/latitude as it loads.

British National Grid
Grid north is up, so the meridians lean toward the pole. The imagery is warped into the grid.