The UK does not count homicides until someone has been arrested, tried, convicted, and the appeals process has been completed. If the process does not go to completion – there is no murder counted. And of course, if the process does complete the “murder” is counted in the year the process completes.
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Working backward from the deliberately skimpy data on the UK’s conviction and successful appeal rates that I have, combined with the “official homicide numbers,” the UK homicide rate is somewhere upwards of 6 per 100,000 population per year.

Bottom line? The actual UK homicide rate is probably more than one third higher than the US homicide rate.

read more: http://extranosalley.com/uk-lower-homicide-rate-than-us/