GridPoint GB

If you have a paper Ordnance Survey (OS) map, GridPoint GB makes it easy to locate yourself — even when you've got no mobile reception.
The app uses your iPhone's GPS and compass to provide your current grid reference and heading, and illustrates how to use this to locate yourself on the map. Coordinate conversions are implemented via OS's OSTN02, accurate to within 1m.
The app also:
* Includes an Apple Watch app and associated complication
* Reports full numerical easting and northing, tetrad, raw latitude/longitude, a nearby postcode, and various heading parameters
* Has a sharing facility so you can share, copy or store your location
* Calculates next sunset and sunrise times
* Can emit a distress signal, just in case
All features are free to use — the in-app purchase is simply a tip you can give the developer to show appreciation.
For details and instructions, please see http://mackerron.com/gridpointgb/
Please note: this app works ONLY in Britain (i.e. England, Scotland or Wales). It does not operate in other parts of the UK or British Isles and cannot provide coordinates in Northern Ireland or the Republic of Ireland, since a different coordinate system is used.