The British are still coming.Boyett Ostar, aka the producing duo Bob Boyett and Bill Haber, have extended their first-look option on possible U.S. transfers of shows from Britain’s National Theater. New agreement runs through the end of 2007. The latest product of the trans-Atlantic marriage is Nicholas Hytner’s production of “The History Boys,” which opens Sunday on Broadway. Next up: “Coram Boy,” staged at the National last winter and based on Jamila Gavin’s novel, which won the Whitbread children’s book award in 2000. The dark tale — infanticide plays a big part — follows two boys in 18th-century England. No timeline for the production has yet been set.