He's not that badly off. I know he isn't earning millions but his tax company has nearly £400k in it. A trip to Nobu wouldn't kill him.
It seems an odd choice for him because I read somewhere about a year ago from a costume designer or some such that he takes very good care of himself. It sounds kind of unhealthy. Also I would have thought that if they genuinely wanted private a better restaurant would have been more so. you see plenty of celebs in nice places in London and people don't photograph them inside the restaurant.
Maybe she just likes ribs and diner type fast food.