The top (larger) hole in a BS 1363 socket is the earth connection.  This is a point which is literally connected to the earth and is there as a safety measure, all exposed metal parts of an electrical device which is plugged into the socket must be connected to this terminal.  UK wiring regulations also require that plumbing pipes, taps and radiators are electrically connected to this same earth.  Touching this earth is therefore safe, providing of course that you are not touching a live wire at the same time.  BS 1363 specifies that it shall not be possible to contact any live part by sticking something into the earth pin hole.