Etiquette of wearing the wrong academic robes at graduation as a lecturer in the UK
In the United States, there are typically two acceptable pieces of regalia to wear: either your university-specific regalia or a "generic Ph.D." regalia that is just simple black robes plus your doctoral hood.  It would seem rather gauche to me to wear non-Ph.D. regalia or to wear the regalia of a school that is not your alma mater, even if that is where your post is.  I don't know if "generic Ph.D." regalia is also accepted in the UK, where many universities have a much longer and more formal set of traditions than in the US, but I would recommend that you look into that as a possibility.