It's been a downward trend from 30-40 years ago. Mid 90s was the start. Our seaside town started going downhill back then, M&S closed down, closely followed by lots of other "High Street" shops, and the terminal decline started. It was all charity and bookies about 20 years ago, and has just got even worse, now, with all the money laundering shops, selling vapes, ethnic foods, nail bars, Turkish barbers, American candy shops, etc. Lots of other towns up here in the North West have been going the same way over the past few years, such as Barrow, Bradford, Bolton, etc. It's not just derelict/run down town centres, there's so much dereliction everywhere - boarded up houses, litter, and so much deterioration in behaviour with low level crime, anti social behavior, illegal parking, etc.
I think people are finally starting to notice now that it's spreading to "naice" places in previously unaffected areas. But nothing will happen to turn the tide until it starts to hit London badly - then suddenly the politicians will wake up because they can't have it happening on their door-step!
It's so sad that the country is becoming so run down - some places are starting to look like third world slums.