This is a picture of Joss Stone:
This is a picture of Joss Stone playing Anne of Cleves:
This is a painting of Anne of Cleves:
It's pretty generally accepted that Anne of Cleves was not a striking beauty, and that this was the main reason Henry VII was unwilling to consummate the marriage and thus sought an annulment pretty quick smart.
Female acting roles seem to almost exclusively be the domain of the beautiful. Couldn't we at least have the historical characters who were renowned for not being conventionally beautiful portrayed by women who aren't stunningly attractive?