Depends on what sort of maps you want. There’s a few options for OS maps, eg Viewranger or Memory-Map. Though you have to buy the maps for these, maybe not cheap if you want a large area.
Or AlpineQuest and MM-Tracker can display maps in QCT (Memory-Map) format, if you get a copy of the maps somewhere…
Also quite a few free apps for OpenStreetMap. eg OsmAnd lets you download offline maps for the whole country.
For batteries, if you have the screen on all of the time it will use a lot of power. But it should be not bad if you just switch it on when you need it. Also if using GPS that will use the batteries more.