You’re posting an article by someone with eccentric views on a lot of topics and an anti-multiculturalist agenda to advance. (For example, they believe that Rishi Sunak is not English.)
One reason homicides are down is because hospitals have got better at keeping stabbing victims alive. Stripped of context it looks like a win. Put in context the question becomes, why are there so many stabbing victims? Gang crime, etc.
Also, hospitals have got better at treating knife injuries in lots of places, not just London. This would not explain why London has a low homicide rate per capita compared to lots of other cities.