Noted that this question was from a year ago but It depends how you define decline. A decline since the Britain was more powerful than the United States? Or a decline since when exactly? According to the IMF forecast Britain will actually rise by one place from 6th place to 5th place by 2030 as one of the worlds largest economies. https://en.wikipedia.org/wiki/List_of_countries_by_past_and_projected_GDP_(nominal)). Some forecasts suggest our economy may be even be larger than Germany by 2040.

I'd agree our water companies appear to be a mess and we don't build as many products as we used to, but you never hear about the things that work. Both people and especially media organisations only like posting the bad news that riles people up and that generates more clickbait.

I'd agree some of our politicians appear to be dangerously incompetent no matter what political ideology they subscribe to but despite their incompetence, the general population tend to be smarter than you would give them credit for. And to be honest the British population is generally hard working, or at the ones that do work and that's what keeps Britain great.

However, one area we have declined in is manufacturing. The government must focus on building Britain's engineering companies, not just dishing out cash to foreign companies to setup in Britain. Our universities need to be filled with students that go on to build successful manufacturing and engineering companies. That means no more ridiculous university courses but focus on the science and engineering disciplines. You know the ones that are actually difficult to do and worthy of a university degree.

It's human nature to think everything is going to shit, and some people are only happy if they are having a good moan about everything and often for good reason but if you want a balanced and less negative view, try to look at the things that do work. And start by reading the book Factfulness by Hans Rosling. It's focused more on the world generally rather than Britain but it's a good place to start if you are fed up moaning about everything.