Not so sure about Britain, honestly. And most of the US's power comes from having (according to Wikipedia) over two thousand nuclear missiles.
Sort of.
Well, yeah, and they spend more money on their armed forces than the next ten or so countries combined.
Definitely not Britain. Their hegemony was dead by the end of the First World War. For that matter, for all that massive conventional forces and nuclear stockpiles are
a source of power, they're less meaningful in the current paradigm than perhaps any other point in history, given the relative importance of soft power these days. That, and our military power isn't really geared for the conflicts we've been fighting since... probably the Korean War or so; a nuclear arsenal or conventional force designed to fight a land war against the Soviets doesn't do much in a highly asymmetrical conflict against an opfor that isn't going to try to fight pitched battles.
Of course, on the flip side, I'm not inclined to listen to the hysterical capitalist economists who are apparently convinced that China is going to grow indefinit- actually, I'll stop there, or I'll still be here twenty minutes later ranting about the sheer shortsightedness of the people who bought into Francis Fukuyama's
laughable declaration of the "end of history" after the Soviet Union collapsed.