This is a bespoke McQueen coat, whose origins are clearly from two RTW coats. Kate blog What Kate Wore notes that the Duchess "mashed together" McQueen's Utility Peplum Coatdress and the Utility Coatdress.
The Utility Peplum Coatdress (grey) is described by McQueen: "Double leaf crepe dress coat, with peplum back pleat and utility pockets; features waist belt, rounded neck without collar and concealed button closure." The coat is 57% acrylic and 43% viscose. Retail: $3,875
The Utility Coatdress (black) is described: "Double leaf crepe coat, with utility pockets, waist belt and pleated back skirt; features rounded neck without collar and concealed button closure." 53% acetate and 47% rayon. Retail: $3,995
The Countess of Wessex has previously worn the Utility Peplum.