None. The purpose of official fuel economy figures is to provide an objective comparison between different models of car. In order to do this, tests must be conducted in a manner which is accurate and repeatable, using a standardised test cycle run in a laboratory. This eliminates variations caused by road type, traffic and weather conditions, driving style etc.
The introductions in the Vehicle Certification Agency’s fuel consumption booklet and the corresponding website state clearly that the official figures might not be achieved under ‘real life’ driving conditions. However they serve as a means of comparing models.