Its an ancient practise, codified into law in 2012 when the regulatory framework was re-codified from multiple laws like Port of London Act 1908 as well as time immemorial acts like this.

According to the article the original practice is medieval, not ancient. It's colloquial usage of "ancient" as in "my car is ancient" is a bit odd.

Fun fact: in English law "time immemorial" has a very specific meaning: it means "any time before 1189". See https://en.wikipedia.org/wiki/Time_immemorial for more.