

“will protect the most vulnerable,"
It’s often useful to imagine that they know what people will think they mean when they say things like that, but that they actually mean an entirely different “most vulnerable” group: MPs, their friends and associates.
Pretty much this. Up to that point, it was Britain and a few other European nations that were doing all the management* in various places in the world. After WWII, they realised: “You know what, we’re tired and worn out and everyone wants us out anyway. We’re going low energy to rebuild at home. Someone else can step in if they want.”
* a.k.a. “Colonialism”. Management is an odd choice of synonym I grant you, but once you’ve got a colony, it’s in your interests to run things in good order. Until the locals rightfully kick you out, that is.