It's like this- London (where I've lived a lot of my life),England, and the rest of the UK are nowhere near as good as people make it out to be. BUT neither are Paris/France or any of Spain, and definitely not Australia.
Having visited a LOT of the UK, I'd have to say the best is London, purely because there are no other cities (bar one, the city that never sleeps) that even compare to it.
Another thing is that there are MILLIONS of people, squeezed into a very small island. English people suffer from overcrowding, and a relatively high immigration rate. Scottish people suffer from wounded pride that England is the richer, more successful part of the island. The Irish suffer from isolation, and the Welsh suffer from being ignored or taken for granted. Which can leave a very poor attitude with some people. Add British cynicism, and you have a potentially offputting mix.
Unlike other countries, we have little to boast, which is why the Olympics are a good thing, because it can bring back some positivity (if we do them right).
Instead of asking opinions, you should visit. The grumpy demeanor drops significantly when you are here, and the UK has billions of fantastic little treats. And, at the end of the day, you can't ignore the fact that Britain once ruled the greatest empire ever-there are signs of this from industrial revolution sights, to a varied and established multiculturalism.
Any details you want, this is the finest place to find it, and is American, so hopefully you trust it as well
https://www.cia.gov/library/publications/the-world-factbook/geos/uk.html