The alternatives to London are Paris (the closest competition as a city qua city), Frankfurt, Amsterdam and Dublin, and I believe they specialize differently.
When I buy EUR-denominated ETFs, they're almost invariably listed in Dublin with IE ISINs. Euronext stock exchange, the largest stock exchange in the EU, is in Amsterdam (domiciled - it's got multiple market places). For financial services, Frankfurt just overtook Paris[1] but they're pretty close.
London's population is almost double all of Ireland. Dublin is not an alternative at all. The annual production of new STEM graduates in the UK is half the population of Dublin alone.
