The Highways Agency pays a private company called TiS (Traffic Information Services) to collect and process traffic data from the motorway and trunk road network. They have a website at
www.trafficengland.com which shows current delays on England's roads (there are similar services for Scotland and Wales too). They also display information about roadworks, incidents, future events, current speeds on Motorways, access to CCTV cameras, and current variable message sign and matrix signal settings. Best thing is, delays are colour coded by severity and shown on a map - just like Google does in the USA.
All the information is collected from the roadside in real-time and is provided free of charge to anyone who wants it. It's all run from a purpose-built facility called the National Traffic Control Centre near Birmingham.
If Google wanted to set up a traffic information service for the UK, nobody is stopping them. They just have to go and ask for the information and it will be given to them on a live feed, at no charge. It's part of the terms and conditions of the contract that the Highways Agency won't charge organisations for the data, as that would take revenue from the likes of TrafficMaster who do have subscription services, and effectively the government would be putting them out of business.
My guess is that there are so many other providers out there that it's not cost-effective for Google to spend time developing the service... Even though they obviously already have a model in the USA, the data is most likely in different formats and from different sources, so there probably isn't a great deal of commonality they could share between the 2 countries. But if there's a will, there's certainly a way!
I tend to use the traffic england site to decide whether to leave work by looking at the current motorway information page, which shows the speeds between every motorway junctions and links to the CCTV cameras so you can see just how bad the jams are before you set off!