Helen Mirren would be better (and tougher!)
To her credit however, before she played the title role in 'The Queen' she stated that she was a republican (in the British sense) and all her family were republicans. However, having researched the role in order to play her Maj, she has a lot more sympathy for her and rebuked someone who insulted the Queen within earshot....
I would much rather see Helen Mirren in the role.