Source please. RBS is the tenth largest bank in the world with $2.5mmmm in assets. London has HSBC and Barclays, but together they are only about twice the size of the Royal Bank of Scotland.
The financial services industry is more than just banks, but in any case London has Natwest, LLoyds/TSB and several others. They also have a controlling interest in RBS, which since the financial crisis is now mostly owned by the British government.