I agree with sp1814. People like Sentamu because he is thought to offer clearer, more outspoken guidance than Williams. But Williams has had to fudge because he leads a hopelessly fragmented congregation, from liberal Americans who think gays should be priests to conservative Africans who think homosexuality is a perversion. (I'm just picking gays as one source of disagreement; there are others.)
If Sentamu speaks forthrightly on this, he's going to outrage one part of the church or another, and quite possibly split the worldwide Anglican communion. I don't know whether UK Anglicans worry about this or not, or which line they would prefer their church to take.