You must not drive faster than the speed limit for the type of road and your type of vehicle.
In an urban area with street lights there is an automatic 30mph limit. In other areas the national limit applies unless a formal order has been made to set a different speed limit. Go to GOV.UK: speed limits.
Experience shows that drivers tend to drive to the character of a road rather than a wrongly set speed limit. Introducing a limit on its own does not mean that speeds will reduce. Driving speed plays a big part in the cause and severity of road accidents.
The speed limit is the absolute maximum – it doesn’t mean it’s safe to drive at this speed in all conditions.