Banks haven't moved HQs out of London, and likely never will when #1 and #2 in the world are New York and London, and share the same language, and want to be as time-zone close as possible.
The pound was the world reserve currency, India was the jewel in the largest empire the world has ever seen and upon which the sun would never set, and Westminster ruled directly and indirectly over more than a third of the world GDP — roughly equivalent to $30 trillion/year against the world of 2018.
When my brother was a child, that same great empire had been so reduced that it found itself forced to make a series of humiliating fishing rights concessions to Iceland, total population ~350,000.
The world can become unrecognisable in even just a few years; that just gets more likely when you assume nothing can ever change.
Back in the early 00's when it looked like Frankfurt might compete with London, Gordon Brown introduced 'light touch regulation', and aligned us strongly with the US which made Frankfurt unattractive to US banks. Of course that fanned the winds of the credit crunch.
Let's hope history doesn't repeat itself post-2020, and Rishi Sunak doesn't do anything like that. But who knows?
When the British took over India it was 31% of global GDP. When they left India's share of global GDP was 2%.