Since WWII, the British economy has developed at a similar pace to the French economy.

Also, was it not more that the industrial revolution made the United Kingdom rich enough to finance the entire military and navy than the other way around?

If Britain falls behind in the next few years, it is more likely due to Brexit and not because it lost some colonies 70 years ago.

I'm British and I broadly agree with your sentiment even if not the specifics. While the UK has a lot of fantastic resources to hand, a lot of its current wealth is built around innovations and capital built up through past glory and privilege than any current day innovation or progress.

Even when the British do come up with something amazing (Arm came from here, after all) it often ends up getting acquired by some overseas company, the talent disappears to the US, or it fizzles out as an overseas alternative takes it to the next level. The US doesn't seem to suffer from such "drains" on its talent and successes.

Despite this, I'm pretty bullish about the English speaking world in general, but the UK is going to struggle to find a place in it in the short term. Canada and the US have NAFTA, Australia and New Zealand are in the new Pacific trading bloc, and the UK is.. well, floating outside the EU with a total idiot in charge, and it's just as well we have that past glory and privilege to ride out a little longer(!)