Not many people know about it, but someone over on the MGRover forums has told me the Highways agency are in the middle of a very snazzy trial of providing realtime Traffic data, including congestion reporting.
The home page is here: http://www.highways.gov.uk/news/m25_rt/index.htm
TrafficMap which shows you England-wide comprehensive information about motorway incidents and what the motorway signs are saying:
Real Time Traffic which shows the current speed and flow information for selected areas. Currently covered are Leeds, Manchester, Birmingham, Kent and M25: