I've heard that story too, but I suspect it relates to the law when the ASLs were originally introduced (1986), not the law as it stands now. It certainly doesn't seem to be borne out by the actual law (Transport Signs Regulations & General Directions 2002, the text is available though Google) which explicitly says that you must stop at the first line except in the case of "a pedal cycle proceeding in the cycle lane" - i.e. it was drafted with ASLs in mind as it explicitly tells the cyclist they can ignore the first line if they're in the cycle lane. So if they'd wanted to permit ASL access from any part of the road they could just as easily have said so. Your guess as to their intentions here in not doing so is as good as mine, but it remains a guess.