Robert Tombs, in fine form here, points out that "Britain is more secure from major external threat than for half a millennium. Taking a long view (say the last three centuries) it remains what it always has been — one of the half-dozen or so strongest states in the world, and one of the most global in its attachments, its vision, and its trade."
That's not too bad, is it? Britain is more of a success over that kind of long view than pretty much anywhere else.
Let's not go overboard about how good things are. After all, self-congratulation is normally unattractive, probably unhelpful and, I'd like to think, rather un-British; while a sort of moaning declinism coupled with a 'they do these things much better in France'-ishness is as about British as a Millwall fan having a curry and a lager.
But I wouldn't want declinism to go too far. A certain quiet self-confidence is not a bad thing in a nation. It makes it a better place to live, for one thing. It might even be useful in current circumstances: "Power is also based on intangibles such as self-confidence, a clear strategy and determination, and here we may be lacking. Russia, with an economy the same size as Spain’s, behaves like a superpower in the Middle East and is treated as one. But we fear we cannot even negotiate a mutually beneficial trade agreement with the EU."
And let's not forget confirmation bias. If you see Britain through declinist spectacles then you'll be spotting all the bad news and doubting any good news that does filter through to you. Why go through life in that frame of mind?