I have Anquet Outdoor map navigator on my phone which I got to replace the sadly missed viewranger. It enables you to download maps to store on your phone (as did viewranger when you bought them). It isn't as good as the OS app as you have to manually switch between 1:50000 and 1:25000 but I can live with that. You can upload a route from a gpx etc and record a track, which it seems to do nicely without draining the phone battery. You pay an annual subscription, get the maps for local storage and get updates 4 times per year.
I got fed up with trying to remember to download the OS maps before I needed them, also I think for those with a memory challenged phone you can't save them to an SD card.
I can't remember whether I looked at memory map and discounted it, or whether I didn't check it out.