There are a lot of people in finance in London, also a lot of big tech (seen the current, and new, Google buildings at KGX?), quite aside from a lot of contractors :)
It's true that there are startups that try to get away with hiring people out of school for peanuts, from all across Europe. They often have a turnover of half the staff leaving within a year and it's no coincidence.
I worked in startups. I've seen first hands employees being abused and leaving in drove after 1-2 years to get significantly more, literally anywhere else. Name any company in London, probably have some ex colleagues there.
That being said, I am talking about developers who can handle a HackerRank test and a coding interview on the whiteboard. It's not a high bar by any means but it is something.
edit: ah, those stats are based on advertised salaries, not actual salaries after the bait and switch - so they're all inflated.
There are 10 000 employees in every building in canary wharf (the financial center with tens of banks). There are a similar amount of employees in the new buildings of Google, Apple and co. Plus multiple large companies but not necessarily as great. That's tens of thousands of roles to fill.
Look at Canary Wharf at lunchtime and tell me with a straight face that those are developers.
Ironically, accountants and finance folks eat at their desks, so if you see people outside at lunch time, they might very well be technology folks. They'd be wearing shirts though, easily confused with business people.