Being from London I can say with authority that most of London is a ****hole and is getting worse by the day.
The east end is and always has been an absolute dump filled the dregs of humanity. South London should be wiped off the map along with everybody who lives there. Parts of North London, like Edmonton, are populated with the filth of the earth and serve no purpose to humanity. West London is bearable but places like Shepherds Bush, Ealing and Acton are places you wouldn't visit out of choice.
Nottingham is depressing place that should be levelled while everybody there is asleep.
But, for real horror, visit Istanbul. **** me, there's some horrible parts of that city you'll leave pretty quickly.