Not that the math would change much or the point wouldn't stand, but there's no point in doing calculations without questioning OP's numbers.

For "UK", they used the 2019 numbers for London only, and only accounted for Murders & Manslaughters.

"London was an exception to the trend of an overall drop in knife killings. The number of homicides – murders or manslaughter offences – involving a knife or sharp instrument decreased by 8% in 2019 to 242 offences." source

And for the US they used the 2020 numbers which are 25% higher than 2019, and also include Unintentional & Defensive homicides, which the numbers for UK London dont't. source