The 80-year-old actress will play the lead role of a grandmother to Oliver Jackson-Cohen, who struggles in her relationship with her gay grandson, reported Variety.
Written by novelist Patrick Gale, the story is based on the relationship shared by his parents.
The series will air next year in June as part of Gay Pride Month, which marks the 50th anniversary of the decriminalisation of homosexuality in England and Wales.
The show also features James McArdle, Joanna Vanderham, Laura Carmichael, Julian Morris, Julian Sands and David Gyasi.