MI6 officer Richard Tomlinson[1], wrote a tell-all autobiography in which he described an HR relationship between the city banks and MI6 whereby officers were 'retired' into city jobs with few questions about the gaps in their CVs. In return the bankers were given some intelligence tidbits as quid pro quo.

I should point out his departure from MI6 was acrimonious, so he's not an entirely neutral source, but interesting nonetheless.

[1] https://en.wikipedia.org/wiki/Richard_Tomlinson

Wall Street, not the City of London, and probably insiders leaking info instead of spooks at work, but I’ll just leave this here:

https://www.cnbc.com/2018/02/14/insider-trading-is-still-ram...

If it's private companies and not governments in a different city in a different country on the other side of the world it's not terribly relevant.

“One study says the close relationship between the government and banking insiders during the financial crisis affected trades.”