Advisory speed limits:
Advisory speed limits are often being used in particularly sensitive areas, such as outside schools, where it is considered that a reduction in speed would benefit the immediate community; they are there to encourage people to drive at a lower speed than they might otherwise do. In common with assessing all other driving situations examiners must assess whether the driver’s actions are safe and not automatically record a fault if the driver does not rigidly comply. As in some instances there may be mitigating circumstances, such as in school holidays or at quiet times of the day, where there are very good fields of vision and low pedestrian activity and safety is not compromised. However, there will be instances where to exceed advisory limits could not be considered as safe and sometimes especially in narrow residential streets the driver may need to reduce their speed considerably lower than the advisory limit.
The above is taken from The Driving Examiners Manual.
Where appropriate it has been reformatted and edited so it only refers to the driving of ordinary cars (class b).