Re: As you might expect...
The annual subscription to the OS app (which includes both Landranger and Explorer maps) is about the cost of two printed OS maps (which you always need for anywhere you are visiting as you can guarantee that it will be on the boundary of two maps!). You can download as much of the map as you wish to your device, so you are not reliant on having a data signal, and you can use the credentials you create your subscription with to log into the OS website, where you can print off as many parts of the map as you like, if you wish to have a printed version with you while you walk.
Unless someone turns off both GPS and the phone system it will always be able to triangulate you and so it is impossible to get truly lost. It will also track your activity if you wish it to so you can see how far you have walked (including vertically, it has the best terrain height information of any mapping app I've used). You can plot routes in advance and then follow them without either ruining your map or discovering halfway around your walk that the pencil you used to mark it has rubbed off.
It also includes an augmented reality function where holding your phone up to the view will tell you precisely what you are looking at.
This is one area where new tech really does beat the old tech hands down.