Don't listen to the complainers here - this episode is worth watching. Like USS Callister it does feel different to most Black Mirror episodes, but that shouldn't be seen as a bad thing.
The black and white cinematography is well suited to this episode which has a feeling reminiscent of The Road - both post-apocalyptic with no mention of what the 'apocalypse' was, both tightly focused on one or two characters.
The episode isn't a neat package but again, this is Black Mirror we're talking about - expecting a clear lesson or meaning out of it is to misunderstand the point of the show. We don't know the details of what led to this apparently post-apocalyptic world, but that doesn't matter. That isn't the focus of the episode. Maxine Peake's almost solo performance as Bella is excellent, and what she was looking for or why is never really relevant. What is relevant is her fear and determination. We know nothing about her, but we feel the tension and desperation. While some episodes of Black Mirror feel like I only observe the characters, with Metalhead I was there with Bella in the desolate unknown countryside, invested in her plight.