Huh? It looks like the story is that Brits might be allowed to create and serve certain warrants. The US Constitution places requirements on warrants, and I don't see that these would be violated by that agreement. If the Brits got warrants that didn't conform with the Fourth, they'd be invalid, just like US warrants that don't conform are invalid. This doesn't violate sovereignity any more than C++'s "friend" violates encapsulation: it voluntarily allows alternatives to the current interface. I'm not an international lawyer, but what such law would it violate? Lastly, the Declaration of Independence is a stirring document showing many of our founding ideals, with absolutely no legal force.