Is it really insane? That is less than 200 per day, in a city of 15 million people.. a single gang of 20 thieves could likely manage that! The real question is, how does this compare to other cities of this size and demographics?

Why compare demographics?

I would expect a city that's mostly retirement communities to have less snatch-and-run thefts than a city where tourists make up a large part of the population at any given time.

Interestingly, Tokyo has a much larger population than London with a very low crime rate. I can’t find specific numbers on stolen phones (only lost phone statistics are published) but robberies/muggings are 0.04 per 1,000 yearly versus London’s 2.83 per 1,000. Seoul has a similar rate to Tokyo, 0.4 to 0.5.

Also, is there even a single city of London’s size that is mostly retirement communities?

Retired people a probably quite easy targets in this scenario.