I mean London has been one of if not the centre of finance for what 300 ish years? Being part of the EU was definitely a plus and losing that has likely caused some business to move away but its not like London was a bit player in finance until it joined the EU in the 70s, seems natural to think it will remain a large player afterwards, if not as large as if it stayed in the EU.
London wasn’t the centre of a global empire in 1960 either, but it was still one of the main global financial centres. The fact that GBP isnt as important as the euro or USD isnt really a factor there as far as I understand (that’s more important to the British government and how much leeway it has in monetary and fiscal policy).