Céline has posted an interview with Henry Pavlovich from the latest edition of The Linguist. He mentions the criteria being discussed.
As this is an achievement and mark of approbation for the profession of linguists as a whole, membership of any ONE approved professional body in the language field (eg IoL, ITI, AIIC) will be sufficient.
This still begs the question of why a charter should be granted to the profession of linguist, which I take to mean language teacher, translator or interpreter (in the USA it would be taken to mean a specialist in linguistics too).
Those who think it would be good to be a chartered translator still think it odd that the IoL should have the charter, since the ITI came into existence, apparently, because the IoL thought the translators’ group in the IoL was getting a bit above itself.