What is the title of this movie?

The movie stars someone like Maggie Smith's (she's the one I kept thinking about, but as it turns out was not her) look and age. It's about her and a black guy having an illicit affair. I am not sure if she was a teacher and the black guy was her student. I have a hunch that this is a British/BBC film, but that's just inference. I was able to catch this up at Cinemax. I hope you guys know this!