I don’t do iPhone, but if ViewRanger for iPhone is the same as Android, then that would get my vote. More so as you can use OSM maps with it for free, which in my opinion these days are generally better than OS. OS may have little church steeples that OSM doesn’t bother with but for paths, bridleways, roads etc, it’s great and people mark trails even that would never show up on OS.
Downside with OSM is you can’t download a whole area in advance for offline use. You can browse the area and scroll around to cache the maps but it’s hassle. OS you buy tiles for whole areas (usually more expensive than a paper map for 1:25000).
Oh, and buy ViewRanger once and you can use it on any supported device, keeping your purchases and tracks.