Ooh! Update. The following error,
```cartopy._crs.Proj4Error: Error from proj.4: conic lat_1 = -lat_2```
suggests the uk plot we are doing is misuse of the projection.

If the secant latitudes can't be like this, that suggests we can only use the projection for polar cones, right?
Otherwise, if you keep rotating the cone towards the equator you'll hit this error...

Thoughts and comments eagerly craved.
Perhaps we should just support north polar plots for now?