Though Sian Phillips's roots are more working class (she's the daughter of a teacher and a cop), I think the word “regal” is the best way to describe her powerful aura. Even if you don't know her by name, she has graced many of my (and possibly your) most favorite things.
She's probably most famous for her role as Livia in the awesome I, Claudius, but she also graces the excellent George Smiley series Tinker Tailor Soldier Spy and Smiley's People, opens David Lynch's Dune with a commanding forehead, accepted and declines invitations in Age of Innocence, and if you're not so much the BBC miniseries or historical romance type, maybe you remember her from Clash of the Titans or Ewoks: The Battle for Endor (which featured a little girl named Cindell who became my little brother's imaginary friend).
She was also once married to Peter O Toole and currently lends her voice to Rufus Wainwright.
But what do you think?