Wow. There is a new button on the top right of the UK Google maps page that shows you either real time traffic conditions on Motorways and Major routes or the ability to pick a time and day and see the predictive traffic speeds.
I have had a short play and it does appear accurate, however the predictions seem slightly optimistic based on checking my route into work on the M25!
The other thing that it does not seem to do, yet, is to give you the option to change the routes it plans based on it new knowledge of the true speed of a section of road.It also seems to have the same gaps in its coverage that the Highways Agency site does so it seems they are pulling their data from there as opposed to TrafficMaster. Still being Google it will have another few years of life in Beta and I am sure it will come. It is certainly a lot better than any of the other options out there.