I've downloaded all of the suggested maps that Forum members have recommended and I have just downloaded the OS app as well.
I use Backcountry OS maps, I paid a one-off payment for it a few years ago. Its always been very good and has never let me down, plus it allows me to download other maps for free like Spain, Italy, USA and so on. If I understand it all correctly if you want to use OS maps on your phone the choice is between a one-off payment like Backcountry vs. a yearly subscription for other OS map apps?
I'm personally sticking with Backcountry because I have already purchased it. I'm also one of those people who has learned not to like subscriptions or three-month trials. I tried a three-month trial before and was annoyed to find that in order to cancel I had to leave the app and cancel it on Google Play. That app is now canceled and I won't fall for that trick again. I have tried out one other app and purchased it for one year because I noticed that this app allowed me to cancel as soon as my payment went through. In 8 months time, I personally will decide if I want to continue with it, it will not be decided by the app or by Google!
When I compare and contrast I have downloaded 4 different apps of France, all use IGN maps and they show all of the walking routes across the country for FREE. My problem now is to decide which one to stick with?