GTraffic.info is just a great all around mashup. It takes current roadworks and traffic information, along with live cameras and mashes it up with Google Maps UK. Google Maps hackers should take note of this mashup for interface ideas to toggle between national and local views.
UK Traffic Advisories is a much less information-rich and currently seems to be in a bit of a test mode. This is a Google Maps UK mashup displaying only limited current traffic incidents.
Both of these mashups are built on the BBC Backstage project.