And military/diplomatic decline follows economic decline as night follows day.
Our seat on the UN Security Council was not given to us by God. It can be taken from us. There is already a case for giving India one. When we were in the EU, we were effectively one of two reps for the economic powerhouse that is the European Union. Now we just represent ourselves.
We have lost our Empire. We have trashed the replacement we found for it. The world is a cold and lonely place. Great power? The Navy is tiny. We have two aircraft carriers we can't really afford, and seem at best to have one in service due to reliability. The Daring class destroyers are notable for their unreliability. It's actually quite scary.
The Army has about 80,000 effectives. After WWI, Germany was limited to 100,000 after the punitive Versailles Treaty and that has been called 'a police force.' When we were in Iraq our soldiers did so much borrowing off the US Army we were known as 'The Borrowers'. I distinctly remember public collections being made for basic items for our troops as things like this were not supplied - by the same Government that was sending them to risk their lives. Tales of soldiers buying kit with their own money abound.
I don't know too much about the RAF, but I do know our allies often do the intercepting of Russian planes on our behalf.
The bottom line is we can't afford anything better. The USSR was brought down because it could no longer keep up with US military hardware due to economic weakness. We are in a similar case.