The HadRM3-PPE-UK data is located on the regional model grid. This is a rotated-pole grid, where the North Pole is re-located. Due the nature of the model there are two different rotated grids used for different variables. The variables page refers to these grids as RG1 and RG2. These names are arbitrarily used to differentiate between the two grids.
Each grid is fully described in its own page with related images and supplementary files: