An Objective-C method to convert British Grid Coordinates (Easting and Northings) to Longitude and Latitude. Converted from Hannah Fry's Python method: http://hannahfry.co.uk/2012/02/01/converting-british-national-grid-to-latitude-and-longitude-ii/

convertToLatLongFromEastings

#include <math.h>
#include "CLLocation.h" //requires Core Location Framework

- (CLLocationCoordinate2D)convertToLatLongFromEastings:(double)E andNorthings:(double *)N {
  
  //E, N are the British national grid coordinates - eastings and northings
  double a = 6377563.396; //The Airy 180 semi-major and semi-minor axes used for OSGB36 (m)
  double b = 6356256.909;
  
  double F0 = 0.9996012717;
  double lat0 = 49 * M_PI /180 ;
  double lon0 = -2 * M_PI /180 ;
  double N0 = -100000;
  double E0 = 400000;            // Northing & easting of true origin (m)
  double e2 = 1 - (b*b)/(a*a);              //eccentricity squared
  double n = (a-b)/(a+b);
  
  //Initialise the iterative variables
  double lat = lat0;
  double M = 0;
  
  while (N-N0-M >= 0.00001) { // Accurate to 0.01mm
    lat = (N-N0-M)/(a*F0) + lat;
    double M1 = (1 + n + (5/4)*pow(n,2) + (5/4)*pow(n , 3)) * (lat-lat0);
    double M2 = (3*n + 3*pow(n,2) + (21/8)*pow(n , 3)) * sin(lat-lat0) * cos(lat+lat0);
    double M3 = ((15/8)*pow(n,2) + (15/8)*pow(n,3)) * sin(2*(lat-lat0)) * cos(2*(lat+lat0));
    double M4 = (35/24)*pow(n,3) * sin(3*(lat-lat0)) * cos(3*(lat+lat0));
    //meridional arc
    M = b * F0 * (M1 - M2 + M3 - M4);
  }
  
  //transverse radius of curvature
  double nu = a*F0/sqrt(1-e2*pow(sin(lat),2));
  
  //meridional radius of curvature
  double rho = a*F0*(1-e2)*pow((1-e2*pow(sin(lat),2)),(-1.5));
  double eta2 = nu/rho-1;
  
  double secLat = 1./cos(lat);
  double VII = tan(lat)/(2*rho*nu);
  double VIII = tan(lat)/(24*rho*pow(nu,3))*(5+3*pow(tan(lat),2)+eta2-9*pow(tan(lat),2)*eta2);
  double IX = tan(lat)/(720*rho*pow(nu,5))*(61+90*pow(tan(lat),2)+45*pow(tan(lat),4));
  double X = secLat/nu;
  double XI = secLat/(6*pow(nu,3))*(nu/rho+2*pow(tan(lat),2));
  double XII = secLat/(120*pow(nu,5))*(5+28*pow(tan(lat),2)+24*pow(tan(lat),4));
  double XIIA = secLat/(5040*pow(nu,7))*(61+662*pow(tan(lat),2)+1320*pow(tan(lat),4)+720*pow(tan(lat),6));
  double dE = E-E0;
  
  //These are on the wrong ellipsoid currently: Airy1830. (Denoted by _1)
  double lat_1 = lat - VII*pow(dE,2) + VIII*pow(dE,4) - IX*pow(dE,6);
  double lon_1 = lon0 + X*dE - XI*pow(dE,3) + XII*pow(dE,5) - XIIA*pow(dE,7);
  
  //Want to convert to the GRS80 ellipsoid.
  //First convert to cartesian from spherical polar coordinates
  double H = 0; //Third spherical coord.
  double x_1 = (nu/F0 + H)*cos(lat_1)*cos(lon_1);
  double y_1 = (nu/F0+ H)*cos(lat_1)*sin(lon_1);
  double z_1 = ((1-e2)*nu/F0 +H)*sin(lat_1);
  
  //Perform Helmut transform (to go between Airy 1830 (_1) and GRS80 (_2))
  double s = -20.4894*pow(10,-6); //The scale factor -1
  double tx = 446.448; //The translations along x,y,z axes respectively
  double ty = -125.157; //The translations along x,y,z axes respectively
  double tz =  542.060; //The translations along x,y,z axes respectively
  double rxs = 0.1502;  //The rotations along x,y,z respectively, in seconds
  double rys = 0.2470;  //The rotations along x,y,z respectively, in seconds
  double rzs = 0.8421;  //The rotations along x,y,z respectively, in seconds
  double rx = rxs*M_PI/(180*3600.); //In radians
  double ry = rys*M_PI/(180*3600.); //In radians
  double rz = rzs*M_PI/(180*3600.); //In radians
  double x_2 = tx + (1+s)*x_1 + (-rz)*y_1 + (ry)*z_1;
  double y_2 = ty + (rz)*x_1  + (1+s)*y_1 + (-rx)*z_1;
  double z_2 = tz + (-ry)*x_1 + (rx)*y_1 +  (1+s)*z_1;
  
  //Back to spherical polar coordinates from cartesian
  //Need some of the characteristics of the new ellipsoid
  double a_2 = 6378137.000; //The GSR80 semi-major and semi-minor axes used for WGS84(m)
  double b_2 = 6356752.3141; //The GSR80 semi-major and semi-minor axes used for WGS84(m)
  double e2_2 = 1- (b_2*b_2)/(a_2*a_2); //The eccentricity of the GRS80 ellipsoid
  double p = sqrt(pow(x_2,2) + pow(y_2,2));
  
  //Lat is obtained by an iterative proceedure:
  lat = atan2(z_2,(p*(1-e2_2))); //Initial value
  double latold = 2*M_PI;
  double nu_2 = 0;
  while (abs(lat - latold)>pow(10, -16)) {
    double swap = lat;
    lat = latold;
    latold = swap;
    nu_2 = a_2/sqrt(1-e2_2*pow(sin(latold),2));
    lat = atan2(z_2+e2_2*nu_2*sin(latold), p);

  }
  
  //Lon and height are then pretty easy
  double lon = atan2(y_2,x_2);
  H = p/cos(lat) - nu_2;
  
  
  //Convert to degrees
  lat = lat*180/M_PI;
  lon = lon*180/M_PI;

  CLLocationCoordinate2D location = CLLocationCoordinate2DMake(lat, lon);
  return location;  
}

You could do this...

- (CLLocationCoordinate2D) convertToLatLongFromEastings:(double)E andNorthings:(double)N {

then

CLLocationCoordinate2D location = CLLocationCoordinate2DMake(lat, lon);
return location;

Updated; Thanks.