I'm not sure this is the most constructive of debates as we should be proud of our great British cities which are flourishing now after some areas went through periods of decline after WW2.
However, for the sake of helping us all move forwards on this debate, I found this graphic from Sun Online. It has used Home Office figures to show the areas where people are most at risk of offences such as robbery, burglary and sex crimes. The article is from June 2017 but the age of the data is not stated.
It puts Greater Manchester 3rd (78.4) and London 4th (75.6).
https://www.thesun.co.uk/news/385025...urglary-arson/
In terms of Sadiq Kahn's impact, he's only been in power for a couple of years so there's an insufficient length of time to benchmark any impact negative or positive he may have made yet so people's views will be just that.