As far as I understand it, the values in the chart should be added to the published altitudes. So, if your DA is 400' AGL @-10 deg C (don't know if Gatwick is, but with the threshold elevation of 200' your CAT1 might be), the correction to be added would be 40' = 440' DA.
Does not apply to operators with approved CAT2 or better, as they will be using their Radio Altimeter.