Basically, the Archbishop of Canterbury is an anti-Moses. When he seeks to use Sharia law in some vague attempt to unify Britain he is trying to use a snake as a walking stick. (The point not being that Islam is reptilian but that it will bite you if you try to lean on it).
But worse than that, Williams would happily lead his people out of freedom into slavery. He holds the misguided understanding that Sharia law can be used as a system of law without bringing on board the inherent assumptions of Sharia law. Whatever we call them, axioms or principles, Sharia law contains assumptions that are integral to it and not negotiable. These are such things as the value of testimony of a woman. A woman's testimony has half the weight of a man's. An infidel's testimony has half the weight of a Muslim's. So the testimony of four Christian women that one of them was raped is only just equal to the testimony of the one Muslim man who (allegedly) raped them. With three Christian women, the charge is unprovable under Sharia law. That is unacceptable to our society.
Sharia law also has strict and often deadly consequences for apostasy, that is, for leaving the Muslim faith. So Rowan Williams would have a British Muslim who becomes a Christian suffer at the hands of his family with the British legal system's full consent.
It is unwise for the leader of the Anglican Church to welcome Islam when he knows so little about it and unacceptable for him to care so little about the believers under his care.