Dan L,
We've been over this a lot in the last couple of days. It IS illegal. The fact that no one bothers about it is irrelevant to your statement.
If you practice discretion then it will continue to be ignored probably, it's not a big deal, but the law still stands. Bear in mind that a schoolboy doesn't set the laws of the land or judge on what is and isn't legal. That's down to the authorities, such as OFCOM (you've been given the links already several times). Readers should take their view as the correct one, not your flawed assertions.
BTW, I'd estimate that 90+% of drivers break the motorway speed limit. Very few are arrested or stopped from doing so. Must be legal then ??