Convert this latitude and longitude into an OSGB (Ordnance Survey of Great Britain) grid reference.

Take a string formatted as a six-figure OS grid reference (e.g. "TG514131") and create a new OSRef object that represents that grid reference. The first character must be H, N, S, O or T. The second character can be any uppercase character from A through Z excluding I.

Create a new Ordnance Survey grid reference using a given easting and northing. The easting and northing must be in metres and must be relative to the origin of the British National Grid.