Before learning that this[1] was a thing, I honestly would have interpreted your remark in passing as figurative rhetoric.

[1] https://en.wikipedia.org/wiki/London_Bridge_(Lake_Havasu_Cit...

> Which legal and financial instruments have defended the City of London itself from being auctioned for parts?

Currently, the Magna Carta. Clause 9, dating from 1297:

THE City of London shall have all the old Liberties and Customs which it hath been used to have. Moreover We will and grant, that all other Cities, Boroughs, Towns, and the Barons of the Five Ports, and all other Ports, shall have all their Liberties and free Customs.

This clause has never been repealed.

But there were other forms of royal charter at least as far back as the Norman conquest, and probably before.