It's not just the US who is declining here. Citing a recent speech from ex-PM John Major,
https://www.bbc.com/news/uk-politics-54879209
I couldn't care less about the 'special relationship', a dire and toadying concept and phrase which is only special to certain folks on this side of the Atlantic. However we
did have a special advantageous role to play as that intersection between the US and the EU which for myriad pathetic reasons we chose to junk. Our NATO membership is one of the few things we have left - that and our islands' geography which even Boris Johnson can't break.
I've always been sceptical about this idea we are a bridge between Europe and America. Given the choice, Britain has always, nearly every government, decided to get completely in bed with the Americans leaving the Europeans out to dry. For better or worse...
Britain has been obsessing about its decline since the 1890s, and in earnest after 1945. Decline isn't absoloute-its illusionary, Thatcher was seen to have reversed Britain's decline and placed the UK firmly back on the international stage. Curiously, Blair, only seven years after Thatcher, was lauded with the same achievement-as was Brown in some ways in his short premiership. What has affected British decline I believe, is the Cameron government's lackadaisical view to defence matters/spending and the lingering reluctance to pursue a more assertive foreign policy after Iraq which affects the Americans too.
Perhaps its because I'm a filthy eurosceptic, but I seriously doubt leaving the EU, in real terms, affects our global standing. Sure, the unfortunate portrayal as Brexit being "right-wing populism" (it isn't) will be viewed negatively by liberal, global personalities and institutions, but lets face it, the EU is hardly a juggernaut in itself. It has no real foreign policy, its millitary might is constrained by existing domestic politics and economics and its main goal in the past 10 years seems to be to avert its own impending doom.
To be honest, I couldn't care less what Major thinks, he was one of the biggest causes for Brexit; if it wasn't for him twisting Thatcher's arm into joining the ERM (against her better judgement), Black Wednesday would not have happened which was a critical source for Britain's sustained euroscepticism and he destroyed his own Party by trying to force through the Maastricht Treaty when it should have gone to a referendum considering most other European countries had one.
We have more than our NATO membership; we have the latent talent in the armed forces, an economy large enough (in theory) to support a strong defence policy, world ties and a stable, reliable democratic system. Sure, we went through a rough patch in the past five years, but if Thatcher and Blair can be credited with turning this country around, I'm sure the future leadership is there that can restore our fortunes once again.