declare namespace gml = "http://www.opengis.net/gml";
      spatial:transform(
        <gml:Polygon srsName="osgb:BNG" xmlns:gml='http://www.opengis.net/gml'>
        <gml:outerBoundaryIs><gml:LinearRing><gml:coordinates>
          278200,187600 278400,187600 278400,188000 278200,188000 278200,187600
         </gml:coordinates></gml:LinearRing></gml:outerBoundaryIs>
        </gml:Polygon>, 
        "epsg:4326")