+1 for viewranger.
You pay for the maps if you want OS.
However I find using the Open Street/Cycle Map is plenty detailed enough, and often has other detail OS won’t.
The OSM stuff however isn’t downloadable as a batch. You have to view the areas in advance and it will cache them offline. Which is a pain.
I only use it for occasional check on an area if I’m not used to it and verify where I am. Though to follow a route I photocopy a paper OS map section, mark it up, and stuff it in my bag.
EveryTrail is worth a look also, mainly because people share routes you can follow, with some summary information, photos and tips. Again I think the maps are cached. OSM again.