I'd be astonished if they changed the terms of the One World agreement if BA are telling their pax they are welcome to use the No1 lounge. For example, not allowing you to guest someone in.
Additionally: Taking people's number to let them know when there is a free space?! It's hardly a top West End London restaurant. If it's full when we fly out in 10 days time we'll simply have breakfast in Jamie's upstairs.