In X Of Swords: Destruction, we saw the death of Betsy Braddock, the new Captain Britain, sister of the old Captain Britain, Brian Braddock Shattered into shards and collected by Saturnyne, Omnibersal Majestrix of Otherworld So the exact nature of her death is unknown but it also led to the reforming of the Captain Britain[...]
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You can see her IMDB entry here. And yes, while James McAvoy is a common factor in a number of these, she is no relation.
So there's no reason to presume that it's James McAvoy who will be starring in the upcoming Captain Britain TV show.
She tweets,
For my 1000th Tweet: Who will be playing Brian Braddock in[...]