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.