Unfortunately no Ordnance Survey product contains any road gradient information. The OS MasterMap ITN data has some supplementary information available as a CSV file which has the locations of extreme gradients that can affect the traffic passable in that area but these are very rare points in the national road network.
You can use GIS Software to calculate slope gradient using the OS Terrain 5 data. You could then use this to calculate an average gradient for any road section you need. This would be a quiet involved process and would recommend seeking local assistance to guide you through it unless you are a confident GIS user.