She has only released two albums (there could some she collabbed on that I’m not aware of): Beth Gibbons and Rustin Man - Out of Season and her recent solo album Lives Outgrown.
It’s not as dark as Portishead and the music is more minimalistic, some of it may be closer to acoustic/folk, but she sings beautifully. If you get the chance to see her live, you should go. During the concert they covered a single Portishead song and the tears were literally streaming down my face.
Another tip: If you like Portishead, you should also give the Goldfrapp album Felt Mountain a listen.