Police in the United States killed more people in March 2015 than police in the UK have killed in the last 115 years.
Here’s how the per-capita math works out:
The US has a lot more gun violence than the UK – the US has about 9000 gun murders per year, and the UK has only about 30 gun murders per year – adjust for population, and we find that the US has about 60 times the gun homicides per capita of the UK.
But this is not even close to the difference in killings-by-police: 12 months * 115 years * ~300 mil/~60 mil = 6900… multiply that by ~3 (the US killed 3 times as many in March 2015 as the UK killed in the last 115 years) and the US has about 20,000 times more killings by police per capita than the UK.
How about per-gun-murder? Back of the envelope math… US police kill more than 300 times as many people per gun murder as UK police kill.
There is way more going on here than just a difference in gun amounts, or gun violence.