Ships get italics... is Slumlord Alley floating on anything? Sounds like it might be (I think I used to live there, is it near Wembley Stadium?). All named vehicles get italics Airforce One, Voyager etc I think the capitals are enough for a house name. If you consider you are quoting the sign you could use quote marks as you have done. Who gets to use one? Generally house-proud or oddly-humoured folk.