Speed limits set the maximum legal speed for a road, but they do not mean it is always safe to drive at that speed. The examiner is assessing whether you choose an appropriate speed for the conditions, and whether you know the limits for different roads and vehicles.
In built-up areas with street lighting, the limit is 30 mph unless signs show otherwise. On single carriageways it is 60 mph for cars. On dual carriageways and motorways it is 70 mph. These are the national speed limits and you must know them without relying on signs.
However, the limit is not the target. In wet weather, your stopping distance doubles. In fog or at night, you must reduce your speed to match your reduced visibility. Near schools, shops, and residential areas, even 30 mph may be too fast for the conditions. The examiner wants to see you read the road and adjust accordingly.
Know your stopping distances. At 30 mph in dry conditions it takes 23 metres to stop, about six car lengths. At 50 mph it takes 53 metres. In wet conditions, these distances roughly double. Keeping a safe separation distance from the vehicle in front gives you time to react and stop safely.