I should add that there is no bliss as sublime as GPS on an OS map. At last you have the infinite detail of the OS but without spending half your walk orienteering instead of enjoying the countryside and your companions.
With BCN, I use the option to leave GPS switched on even when the phone (or tablet) is turned off: it uses less battery than you might imagine and means there's no waiting when you consult the map.
(MAPS.ME is the equivalent for town use. Worldwide, offline and only a one-off payment.)