Back Country Navigator here, you can trial it for 3-4 weeks for free. It costs a one off £6.99 (you can have it on multiple devices) and you get full 1:25k OS maps. You can download on to your device the sections of map you require for using out of range, it will also work if you have data connection. I’ve trialed both BCN and Viewranger, Viewranger did seem a bit better presented, but for £6.99 BCN appeared a bit of a no brainer. Been using it for about 6 months, only drawback is, for non techy person like me, on line instructions aren’t that great and some ‘beyond basics’ stuff I’m finding difficult to work out, but has been great for navigating, downloading others routes to follow and recording our routes.