The author has seriously over-simplified things. Most Londoners don’t think of the north as ‘tony’ and the south as ‘down and out.” (Unless you’re speaking with someone who lives in Belgravia!) There are plenty of great neighbourhoods in South London; and some pretty lousy ones in the North.

It’s more simply that you are either north of the Thames or South of the Thames; it’s more a geographical divide – Londoners aren’t known for being spontaneous so if you live on opposite sides of the river, you aren’t likely to get people over for your dinner party!