In the event of a seat becoming vacant in the UK Parliament, a warrant would be issued by the Speaker of the Commons followed by the issue of an election writ.  During a Parliamentary session the warrant is issued after a motion in the House Of Commons.  If a vacancy occurs after a new parliament, (after a general election) the warrant would not be issued until after the time for an election petition had expired.  If a writ is pending the warrant will not be issued until the petition has been heard.