OutdoorActive does allow maps to be saved offline on the phone beforehand as described on their
help page.
I don't use OA myself but I've read extensive discussions about it in the context of ViewRanger replacement, like so many apps it suffers from gross "featureitis". It amazes me that people make life so difficult with all these features, the last thing I want on a hike is faffing about with unnecessary complicated tech. My view seems identical to yours: I just want to check my exact position occasionally on the OS map, along with the route line if I've stored one, and that's it. Keep it simple!.
Ploughing my own furrow as always, I have achieved this using MM Tracker that uses the Memory-Map QCT format maps - this assumes you have the maps to begin with...
I have the 1:25K maps permanently on the phone.