United Kingdom


EPSG:27700 with TOWGS84 (7-parameter transformation) defined by Ordnance Survey (as EPSG:1314).
More info about systems in use in UK is at:
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guidea.html 

PROJCS["OSGB 1936 / British National Grid",
    GEOGCS["OSGB36",
        DATUM["OSGB36",
            SPHEROID["Airy 1830",6377563.396,299.3249646,
                AUTHORITY["EPSG","7001"]],
            TOWGS84[446.448,-125.157,542.06,0.1502,0.247,0.8421,-20.4894],
            AUTHORITY["EPSG","6277"]],
        PRIMEM["Greenwich",0.0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.017453292519943295],
        AXIS["Geodetic latitude",NORTH],
        AXIS["Geodetic longitude",EAST],
        AUTHORITY["EPSG","4277"]],
    PROJECTION["Transverse_Mercator",
        AUTHORITY["EPSG","9807"]],
    PARAMETER["central_meridian",-2.0],
    PARAMETER["latitude_of_origin",49.0],
    PARAMETER["scale_factor",0.9996012717],
    PARAMETER["false_easting",400000.0],
    PARAMETER["false_northing",-100000.0],
    UNIT["m",1.0],
    AXIS["Easting",EAST],
    AXIS["Northing",NORTH],
    AUTHORITY["EPSG","27700"]]


The definition with 3-parameter transformation:

PROJCS["OSGB 1936",
    GEOGCS["OSGB 1936",
        DATUM["OSGB_1936",
            SPHEROID["Airy 1830",6377563.396,299.3249646],
            TOWGS84[375,-111,431,-0,-0,-0,0]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    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]]

There is also definition merged with the Newlyn height: EPSG:7405

For higher accuracy transformation you can use the grid-shift files described here: http://lists.maptools.org/pipermail/proj/2006-November/002658.html

More information about different coordinate systems used in UK have a look at the tools


PROJCS["TM65 / Irish Grid",
    GEOGCS["TM65",
        DATUM["TM65",
            SPHEROID["Airy Modified 1849",6377340.189,299.3249645999944,
                AUTHORITY["EPSG","7002"]],
             TOWGS84[482.5,-130.6,564.6,-1.042,-0.214,-0.631,1.6810581709138355],
            AUTHORITY["EPSG","6299"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4299"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",53.5],
    PARAMETER["central_meridian",-8],
    PARAMETER["scale_factor",1.000035],
    PARAMETER["false_easting",200000],
    PARAMETER["false_northing",250000],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","29902"]]
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