I'm looking to see if there's an RFU document that matches referee level and required competencies.
i.e. level 9, identifies players leaving their feet
level 8, identifies and attempts to correct players leaving their feet,
level 7, proactively manages players in order for them not to leave their feet
level 6, recognises the difference between players leaving their feet illegally and in an attempt to obtain clean ball.
Basically that type of thing for all the facets of the game.
I really hope that such a document doesn't exist.
As referees, especially at the ground-roots level, we can't be perfect yet the players are supposed to act as if we are. Similarly, even at the elite level, referees sometimes are either ignorant over overwhelmed by facets where they are insecure (the scrum in particular used to be a problem).
I guess my point is that we have 5 levels:
IRB1 = ELRA is a one day course. This is all I have myself! We do the best we can
IRB2 is the norm, 2 day course, self-evaluation/reflection and external assessment
IRB3 is by invitation only, multiple sessions, remedials, monitoring, coaching, etc.
IRB4/5 are the elite/international levels, semi-pro or nowadays even pro.
We are all just as good as each other at seeing the clear and obvious stuff if it happens right in front of us.
We are all just as bad as each other when we miss something important clear and obvious, and none of us are perfect enough to see every offence, especially when we don't have the advantage of a team of three.
And more than that, the 'worse' we are (physically, perhaps not fit enough to position ourselves perfectly), the more we have to proactively manage the players. I am better at managing the scrum than most international referees, because I not only know what to look for, and also have played from 1 to 3 myself, but above all because I simply manage most of the time in advance.
Any referee wanting to 'rise up the ranks' needs to improve from 'good' to 'very good' on all areas of the game, but there simply cannot be any hard and fast guidelines. Per your example, I am level 7 in most games, but occasionally worse than level 9, for this specific issue you chose as an example.
It just isn't that easy