You could try the .NET library Proj.Net. See the Loading a projection by Spatial Reference ID page for how to add in the GB National Grid.
Example code from this forum post:
CoordinateSystemFactory c = new CoordinateSystemFactory();
ICoordinateSystem target = c.CreateFromWkt("PROJCS[\"OSGB 1936 / British National Grid\",GEOGCS[\"OSGB 1936\",DATUM[\"OSGB_1936\",SPHEROID[\"Airy 1830\",6377563.396,299.3249646,AUTHORITY[\"EPSG\",\"7001\"]],AUTHORITY[\"EPSG\",\"6277\"]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",\"8901\"]],UNIT[\"degree\",0.01745329251994328,AUTHORITY[\"EPSG\",\"9122\"]],AUTHORITY[\"EPSG\",\"4277\"]],PROJECTION[\"Transverse_Mercator\"],PARAMETER[\"latitude_of_origin\",49],PARAMETER[\"central_meridian\",-2],PARAMETER[\"scale_factor\",0.9996012717],PARAMETER[\"false_easting\",400000],PARAMETER[\"false_northing\",-100000],UNIT[\"metre\",1,AUTHORITY[\"EPSG\",\"9001\"]],AUTHORITY[\"EPSG\",\"27700\"]]");
ICoordinateSystem source = c.CreateFromWkt("GEOGCS[\"GCS_WGS_1984\",DATUM[\"D_WGS_1984\",SPHEROID[\"WGS_1984\",6378137,298.257223563]],PRIMEM[\"Greenwich\",0],UNIT[\"Degree\",0.0174532925199433]]");
CoordinateTransformationFactory trf = new CoordinateTransformationFactory();
ICoordinateTransformation tr = trf.CreateFromCoordinateSystems(source, target);
double[] point = new double[] {-4.0, 55.6};
double[] result = tr.MathTransform.Transform(point);
Console.WriteLine(result);
Alternatively the Ordnance Survey have a downloadable tool if you register at http://gps.ordnancesurvey.co.uk/convert.asp
Best Answer
If you use the identify tool to click anywhere on the map, you can change the type of units displayed in the location bar. There is a (very tiny!) button at the end of the location bar that when clicked will open a menu displaying a list of unit types to display. Simply select Decimal Degrees or D/M/S to get your lat/long.
edit: Here's a screenshot:
Another simple option is to switch your display units to Decimal Degrees (Data Frame Properties, General tab - set Display: to Decimal degrees). Then you can see the units in DD displayed at the bottom/right as you move the mouse cursor.