I, too, prefer to put "whole" offline maps on my phone, rather than messing around with fiddly caching, then finding out you haven't actually brought some of the bits with you. As I always have the relevant paper map with me, the OS Maps App used to be no-brainer, as you could download what you bought and use it in the App, as well, for no extra charge. Since OS ruined all that by wrecking their own App, I've taken to buying swathes of OS maps separately for Viewranger, usually at 1:50000, which is better IMO on a small device and is a reasonably cheap option, while carrying the appropriate 1:25000 paper map(s) with me.