It's kind of a once-in-a-lifetime thing, and plenty of normal people would do it for a very special occasion (think Mum's 60th Birthday, Golden Wedding Anniversary, etc). Ordinary people do have the ability to save for a very special event. They have five sittings a day (including one bizarrely at 7:30pm) and they're fully booked for the foreseeable future, so it must be popular.
I don't often spend £600 on dinner, but when my parents retired I took the family out for a top class family meal to celebrate that cost close to £600. Not something I'm likely to do again, but absolutely worth it for such a special occasion.
That said, throwing a party at The Ritz (like Monsieur Farage did) is certainly out of most people's price range.