It was actually included in the Standard Code of Rules for all Leagues. It is, I believe without checking, 1b in the list of competition rules, that book referees consult to see [...]
I have two such books, one from the league (Amateur Football Combination) and one from the CFA (Middx). The one from the league has two sets of competition rules, one for the league and one for league cups.
I cannot find any reference to this directive in either of those books, and I have spent the last 30 minutes looking. It definitely isn't 1b in any of the three sets of rules. Am I missing something?