But there are other reasons:
- The Americans and the Japanese. Their bankers and traders like to live in London. It is a big city and easy to adapt. Paris is fine for weekend trips, but you wouldn't want to live there. Hard to settle in, few international schools, and then there is the French
- Language: everybody learns English at school these days. The English, well some of them, learn French, but for the rest of the world, it is a second choice. OK, they could do business in English, but English speaking support staff come at a price in France, while spoken and written English comes as standard in London.
- Legal systems. London contracts are written under English law. Sometimes they might agree to US law, but except in particular circumstances, usually some common law based system, not codified European legal systems. Which means that a large French financial centre would require lots of trained Anglo-Saxon lawyers. Cheaper to get the real thing in London.