My choice would have been Emma Thompson.
Both Emma and Meryl are lfetists whose politics are far from those of Mrs. Thatcher's.
This is going to be a 'Hatchet Job' on her and Reagan as well which is why I have not spent a $ on a movie in the theater in years.
Helen Mirren was astounding as Queen Elizabeth II. I wonder if she could have played Lady Thatcher? That said, Streep has an uncanny ability for morphing accents of every type imaginable (check her out in her early films playing a Pole, a South African, etc). She is a pro and perhaps the best actress in Hollywood. I believe she will do a great job personally.
And SHE can act.