It might be interesting to look up the ratio of police/ citizens in both cities? I would not be surprised if NY had a higher ratio.
The statistics I saw quoted were that NYC has 25% more police officers than London (and London is slightly more populous). This was in the Mail so I can't be 100% certain of that.
Besides the incredible small sample (comparing just a couple of months), another factor will be that NY has decrimininalised marijuana use (provided it's not in public view). My guess (and it's purely supposition) is that a large number of London deaths are drug-related