Can convert WGS84 Lat/Lon to OSGB Eastings/Northings and vice-versa. Credit for original work goes to Jonathan Stott

LatLonConversion.cs

using System;

public class LatLonConversion
{
    private LatLonConversion() { }

    private static double Deg2Rad(double x)
    {
        return x * (Math.PI / 180);
    }

    private static double Rad2Deg(double x)
    {
        return x * (180 / Math.PI);
    }

    private static double SinSquared(double x)
    {
        return Math.Sin(x) * Math.Sin(x);
    }

    private static double TanSquared(double x)
    {
        return Math.Tan(x) * Math.Tan(x);
    }

    private static double Sec(double x)
    {
        return 1.0 / Math.Cos(x);
    }

    private static LatLon Osgb36ToWgs84(double lat, double lng)
    {
        var airy1830 = new RefEll(6377563.396, 6356256.909);
        var a = airy1830.Maj;
        var eSquared = airy1830.Ecc;
        var phi = Deg2Rad(lat);
        var lambda = Deg2Rad(lng);
        var v = a / (Math.Sqrt(1 - eSquared * SinSquared(phi)));
        const int h = 0; // height
        var x = (v + h) * Math.Cos(phi) * Math.Cos(lambda);
        var y = (v + h) * Math.Cos(phi) * Math.Sin(lambda);
        var z = ((1 - eSquared) * v + h) * Math.Sin(phi);
        const double tx = 446.448;
        const double ty = -124.157;
        const double tz = 542.060;
        const double s = -0.0000204894;
        var rx = Deg2Rad(0.00004172222);
        var ry = Deg2Rad(0.00006861111);
        var rz = Deg2Rad(0.00023391666);
        var xB = tx + (x * (1 + s)) + (-rx * y) + (ry * z);
        var yB = ty + (rz * x) + (y * (1 + s)) + (-rx * z);
        var zB = tz + (-ry * x) + (rx * y) + (z * (1 + s));
        var wgs84 = new RefEll(6378137.000, 6356752.3141);
        a = wgs84.Maj;
        eSquared = wgs84.Ecc;
        var lambdaB = Rad2Deg(Math.Atan(yB / xB));
        var p = Math.Sqrt((xB * xB) + (yB * yB));
        var phiN = Math.Atan(zB / (p * (1 - eSquared)));
        for (var i = 1; i < 10; i++)
        {
            v = a / (Math.Sqrt(1 - eSquared * SinSquared(phiN)));
            double phiN1 = Math.Atan((zB + (eSquared * v * Math.Sin(phiN))) / p);
            phiN = phiN1;
        }
        var phiB = Rad2Deg(phiN);

        return new LatLon(phiB, lambdaB);
    }

    public static LatLon ConvertOsToLatLon(double easting, double northing)
    {
        var airy1830 = new RefEll(6377563.396, 6356256.909);
        const double osgbF0 = 0.9996012717;
        const double n0 = -100000.0;
        const double e0 = 400000.0;
        var phi0 = Deg2Rad(49.0);
        var lambda0 = Deg2Rad(-2.0);
        var a = airy1830.Maj;
        var b = airy1830.Min;
        var eSquared = airy1830.Ecc;

        var n = (a - b) / (a + b);
        double m;
        var phiPrime = ((northing - n0) / (a * osgbF0)) + phi0;
        do
        {
            m =
              (b * osgbF0)
                * (((1 + n + ((5.0 / 4.0) * n * n) + ((5.0 / 4.0) * n * n * n))
                  * (phiPrime - phi0))
                  - (((3 * n) + (3 * n * n) + ((21.0 / 8.0) * n * n * n))
                    * Math.Sin(phiPrime - phi0)
                    * Math.Cos(phiPrime + phi0))
                  + ((((15.0 / 8.0) * n * n) + ((15.0 / 8.0) * n * n * n))
                    * Math.Sin(2.0 * (phiPrime - phi0))
                    * Math.Cos(2.0 * (phiPrime + phi0)))
                  - (((35.0 / 24.0) * n * n * n)
                    * Math.Sin(3.0 * (phiPrime - phi0))
                    * Math.Cos(3.0 * (phiPrime + phi0))));
            phiPrime += (northing - n0 - m) / (a * osgbF0);
        } while ((northing - n0 - m) >= 0.001);
        var v = a * osgbF0 * Math.Pow(1.0 - eSquared * SinSquared(phiPrime), -0.5);
        var rho =
          a
            * osgbF0
            * (1.0 - eSquared)
            * Math.Pow(1.0 - eSquared * SinSquared(phiPrime), -1.5);
        var etaSquared = (v / rho) - 1.0;
        var vii = Math.Tan(phiPrime) / (2 * rho * v);
        var viii =
          (Math.Tan(phiPrime) / (24.0 * rho * Math.Pow(v, 3.0)))
            * (5.0
              + (3.0 * TanSquared(phiPrime))
              + etaSquared
              - (9.0 * TanSquared(phiPrime) * etaSquared));
        var ix =
          (Math.Tan(phiPrime) / (720.0 * rho * Math.Pow(v, 5.0)))
            * (61.0
              + (90.0 * TanSquared(phiPrime))
              + (45.0 * TanSquared(phiPrime) * TanSquared(phiPrime)));
        var x = Sec(phiPrime) / v;
        var xi =
          (Sec(phiPrime) / (6.0 * v * v * v))
            * ((v / rho) + (2 * TanSquared(phiPrime)));
        var xii =
          (Sec(phiPrime) / (120.0 * Math.Pow(v, 5.0)))
            * (5.0
              + (28.0 * TanSquared(phiPrime))
              + (24.0 * TanSquared(phiPrime) * TanSquared(phiPrime)));
        var xiia =
          (Sec(phiPrime) / (5040.0 * Math.Pow(v, 7.0)))
            * (61.0
              + (662.0 * TanSquared(phiPrime))
              + (1320.0 * TanSquared(phiPrime) * TanSquared(phiPrime))
              + (720.0
                * TanSquared(phiPrime)
                * TanSquared(phiPrime)
                * TanSquared(phiPrime)));

        double phi = phiPrime
                     - (vii * Math.Pow(easting - e0, 2.0))
                     + (viii * Math.Pow(easting - e0, 4.0))
                     - (ix * Math.Pow(easting - e0, 6.0));

        double lambda = lambda0
                        + (x * (easting - e0))
                        - (xi * Math.Pow(easting - e0, 3.0))
                        + (xii * Math.Pow(easting - e0, 5.0))
                        - (xiia * Math.Pow(easting - e0, 7.0));

        var lat = Rad2Deg(phi);
        var lng = Rad2Deg(lambda);

        // convert to WGS84
        return Osgb36ToWgs84(lat, lng);
    }
}

public class RefEll
{
    public double Maj { get; set; }
    public double Min { get; set; }
    public double Ecc { get; set; }

    public RefEll(double major, double minor)
    {
        Maj = major;
        Min = minor;
        Ecc = ((major * major) - (minor * minor)) / (major * major);
    }
}

public class LatLon
{
    public double Latitude { get; set; }
    public double Longitude { get; set; }

    public LatLon()
    {
        Latitude = 0;
        Longitude = 0;
    }
    public LatLon(double lat, double lon)
    {
        Latitude = lat;
        Longitude = lon;
    }
}