Sadly Bing seems to have stopped serving tiles for OS maps (besides their Web interface). I don’t know if there will ever be a work-around for this, but at least for now it means no free off-line OS mapping for Locus on Android. So even I am feeling tempted to undo the padlock on my wallet on fork out for one of the excellent packages featured up-thread.
In the meantime I down-loaded GB maps from OSM in vector form. This was pretty big (about 0.5 GB), but covers the whole of GB (other countries are available too):
Because it’s vector-based and the rasterization is done on your device, you can choose different themes. Also the night mode is reasonably effective.
Of course OSM is not as consistent as, for example, OS mapping. It’s only as good as the contributions people make to it. But that means when you make a track of a useful path as well as contributing it to the SOTA mapping project you can put it on OSM.
Here’s what it looks like in Locus Pro: