In those situations, just download offline maps to use?

You assume that one knows that the place where you're going is beyond the reach of cellular service before one even goes there.

With apps like OsmAnd and Organic Maps, I can download the map for the entire country and navigate off it. Highly recommended — both on F-Droid.

Can OsmAnd actually calculate routes?

I like having OSM sometimes for, like, figuring out parking and trails in parks, but I've never succeeded using it to get directions on roads.

With Google Maps I can literally download maps of the entire UK, and they are kept locally for 6 months? A year? I can't remember how long.

Any time I'm driving somewhere reasonably far (or where I know there's no service), I'll download or update the maps. Never had an issue.

You don't need the connectivity all the time; it is enough when calculating the initial route. After that, GPS signal is enough, and can be intermittent (in tunnels or deep valleys, for example).

But I've seen another uses of dedicated GPS appliances: bikers. They use rugged ones, they don't break so easily as phones.

mapy.cz works well for this and even supports android auto.

google maps work quite well with offline data in airplane mode.