I think your instructor was talking about making progress on NSL roads. I've done quite a bit of further training and none of the instructors would advise to overtake a car doing 30 in a 30 (or any other posted limit for that matter). It's just taking the pi$$.
Apologies, I did not put that across properly Macduff - my old instructor was actually talking about showing confidence by making progress etc and driving 'at' the speed limit as it's the only way to show the examiner that you know what the speed limit is....clearly he was not telling me to exceed the limit and overtake as none of his pupils would ever pass the test if they did that!
Interesting that you have not commented on bikes overtaking.........