The readings are always measured values SMETS electricity and gas meters occasionally produce high-values which are not actual recorded consumption. They are error codes used to report some kind of meter fault. You need to trap and handle these when processing the data. As the linked documents note: “Due to the nature of the national programme and the position taken by government and the regulator this is no obligation or requirement for manufacturers to publish these error codes so they can be captured and processed explicitly.“
Oh jeez, this would be such an awful thing to learn on a project