Incorrect. You're comparing apples to oranges. The UK only counts as murders or homicides those crimes which have a conviction; the US counts all deaths which are ruled a homicide. Additionally, the UK counts homicides as occurring when the trial is complete; a murder today may not be counted until 2016, and will be included in the 2016 statistics.
The correct proportion is probably somewhere around 33% or 75% - that is, the US homicide rate is about 33% higher than the UK homicide rate. But given the large number of deaths in the UK with an uncoded cause of death, the homicide rate could very easily be equivalent.
Hmm, do you have a citation that the UK does that? Note that in that case, the difference between the US and UK effective rates will be even smaller, so the difference will be even less than a month. That makes the conclusion that this isn't a major impact on differences in lifespan even stronger.
Of course, it turns out they're actually last among developed nations in real health outcomes.