ViewRanger is always voted the tops in similar threads on here, but it is not available on my prehistoric Windows Phone.
Reading your post carefully, perhaps all you want is an app that will show your current grid reference?
Virtually the only app with OS mapping I can get on my phone is one called GeoGPS, this displays your grid ref and additionally your position on a map, but, and it's a big but, it needs wifi to work!
This leaves me with a far better app for giving my position, which is GPS to Grid Ref. I have no idea if it is available for Android and iPhone. It works from the phone's built-in gps and it will work anywhere, even when there is no signal. When you do have a signal it will additionally show your position on an OS map, but I have found that's not a lot of use because when you pinch out the map to see it better, it moves off centre.