I'm very old school when it comes to offline mode - if I'm going somewhere where I need a map (e.g. up a mountain) I'll either buy the relevant OS waterproof map, or print out a map and carry it in a waterproof case and use with a compass.

Edit: I don't know if anyone has died in the Scottish mountains because their smartphone battery ran out but I've certainly been in plenty situations where if I didn't have some kind of navigational aid I'd have been in serious trouble!