I have not finished season 1 yet, but I am already absolutely loving it. Besides what is already said in the other reviews, I love how the major as well as the minor characters react to situations. It's so organic, it's almost as if there was a psychologist among the writers.
While some series excel at the shock value of how extraordinarily some people react to situations, e.g. breaking bad, Unforgotten excels at keeping everyone very very real.