It is a little bit of both influencing each other, but the system in London is definitely the killer app. My understanding of the history is that there are systems that have been entrenched in the London financial industry going back centuries due to the unique history of the City of London and how it administrates itself separately from the rest of England/ GB. It was the global hub during pax brittania, and remained the financial hub of the world after the war thanks to the Eurodollar. It was at this point that tax systems were devised involving their crown dependencies to obfuscate everything which made it a popular place to launder money. A lot of the opulence has developed along side over all these years.

So it was the case that London was the city of the world during the empire which made it the candidate for running the Eurodollar system to rebuild Europe which cemented its place as the financial capital of the world up until America financialized their economy in the late 70s, its colonies allowed for unique tax systems which made it even more popular.

I got most of this from the book entitled Treasure Islands: Tax Havens and the Men Who Stole the World by Nicholas Shaxson, and the documentary https://www.youtube.com/watch?v=np_ylvc8Zj8 that was inspired by it.