What about the big British actors like Helen Mirren (sp?) or Judy Dench? I would say Emma Thomson or Maggie Smith but having both played witches before they might be a bit familiar (it would suddenly feel like watching Nanny McPhee or Professor McGonagall (sp? again :-P) chattin to Nanny Ogg - confusing!).

And the books say Esme is actually quite striking.

And Angeline Jolie? Whaaaaaaaat? :-P