I’m not entirely sure if the Primate of All England can legally make such a decision on his own. Maybe he can. While Anglican bishops posses great spiritual authority, they cannot exercise absolute administraive authority. Archbishop Williams can excommunicate individual members of his diocese on a case by case basis, however he cannot unilaterally alter the nature of the relationship between the Church of England and the Episcopal Church. Such a move may require assent from the Governor General of the Church of England (Queen Elizabeth II) and an act of the Church’s convention (commonly known as Parliament). The Queen prefers to stay out of controversies, and the last thing anyone wants is to see the future of the Anglican Communion debated in the House of Commons.
Interesting. But, then, the nature of the link between TEC and the Archbishop of Canterbury is spiritual; that’s what “in communion” means. The ABC has no administrative authority over TEC at all. It’s a voluntary association and it’s wholly spiritual.