There is often debate as to whether they are legally enforceable. So here is my take on the issue. The Highway Code (Section 171) seems to be clear that they are. As this web page lays out, under Section 36 of the Road Traffic Act 1988 encroaching motorists can have a fixed penalty of £60 imposed and an automatic 3 points added to the offender's licence.
Under the same section cyclists running red lights can have a fixed penalty of £30 imposed.
Are these laws ever enforced? I mean other than warnings in 'awareness campaigns'. I am told that there are at least six police officers operating one day a week in the city to enforce ASLs and report cyclists ignoring red lights. This resource has been put in place until November 2012.
I'm looking for on street evidence of this.