In a nutshell as I understand it you want a free, phone based navigation app that will give you access to OS maps off line for free?
The way I tackle this is to use VewRanger which is free to download and use. This will give maps based on open sources , which can be stored for offline use. But to do this with OS maps you need to pay a premium subscription. However the premium subscription can be bought with credits rather than cash. So then the issue is how to get credits. Well, as a keen walker, who already records his routes, you can convert the gps trace of a walk to a route on VR and publish it for others to use. When you publish a route, you can decide how many credits the route is worth. If anyone downloads one of your routes, the credits can be added to your account and you, in turn, can use these to pay for your OS subscription.
This way you are giving something back to the walking community and in return, they are subsidising your walking to create more routes.
I used just such a method to pay for a month's subscription to LINZ maps of New Zealand on my last trip out there. Currently, I have just over 440 routes published on VR and I charge the minimum, short of free, to download one of my routes - I think it equates to 10p. It you want to have a look, my VR shortcode is nthce