Isn't she the host because she's the Queen, AND he's the heir to the throne behind his father, AND members of the royal family, even greatly extended royal family, must ask her permission to get married?
For example - when Caroline of Monaco married her current husband, he had to ask Queen Elizabeth if he could marry Caroline, since he is a member of the Queen's greater extended family (a German member, not sure how that worked, but I remember reading it in more than one place) and he was a Protestant and Caroline is a Catholic (or was, dunno if she converted).