I still use paper OS maps because I just like having a collection of them, but for android I also use OsmAnd, which uses open source mapping, which a few years ago was terrible, but now is quite often better than OS maps. Downloading for offline use is easy (and free), and places like Surrey Hills not only have all the trails marked, but quite a lot are named too, so its great for exploring new trails. It is great for holidays too, where you can use it in flight mode and the battery lasts forever, I was amazed at the quality of mapping in Germany recently.