They're divorced and he's engaged. SARAH BRIGHTMAN was granted a divorce from ANDREW LLOYD WEBBER yesterday in London after six years of marriage. Also yesterday, Mr. Lloyd Webber announced his engagement to MADELEINE GURDON , a clothing designer. They expect to marry in Britain in February. Meanwhile, Ms. Brightman, who starred in her husband's musical, "The Phantom of the Opera," is scheduled to take the leading role in December in the Broadway production of another, "Aspects of Love."