I totally agree with you that the Queen Mother's charms are quite hard to reproduce. She had beautiful features and smiles gloriously, not for nothing was she called "the smiling duchess". Helen Mirren was absolutely right and played the Queen effectively. Helen Bonham-Carter was more a pretty leading actress in the Merchant-Ivory gerne ("Howard's End", "A Room With A View", "Where Angels Fear To Tread"...etc). Her other stuff did not impress me (even Sweeny Todd). I am sure she would be effrective, but I would not expected to be knocked off my socks. Colin Firth on the other hand (borrow a phase from David O Selznick) "looks damn good" !