LONDON — Robert Phillis, deputy director general of the BBC and head of its commercial arm, BBC Worldwide, is leaving the pubcaster to become chief exec of the Guardian Media Group, owner of the Brit broadsheets the Guardian and the Observer.Phillis has of late been the key exec behind the BBC, expanding sales of its programming overseas and preparing for digital and satellite broadcasting. He leaves the BBC at the end of November but will continue to serve as a consultant to Worldwide. His departure is seen by some observers as linked to dissatisfaction with being No. 2 to Beeb director general John Birt, who is believed to have restructured parts of the BBC — such as the global radio arm BBC World Service — with little consultation with Phillis. In his career, Phillis has been accustomed to calling the shots, first as the head of ITV network company Carlton Communications, and then heading ITV and Channel 4 news provider Independent Television News. He joined the BBC from ITN to become deputy director general in 1993.