Of course, it turns out they're actually last among developed nations in real health outcomes.
The U.S. ranks #1 among 17 affluent, western countries in the percentage of people aged 5 to 34 who rate their health as good. Unfortunately, when doctors look at people’s actual health, at indicators such as obesity, diabetes, and simply the chance that someone will die before his or her next birthday, the U.S. ranks last: young Americans are #17 out of 17 in real health.
Correction: Apparently the UK has trials (inquests, but in cases where they affect public safety, such as possible homicides, a jury is summoned? Or maybe it's passed on to the Crown Court? It's difficult to trace the laws down, but it does appear there is a substantial legal process) to determine cause of death in suspected homicides. So I misinterpreted the statements about the completion of trials there. See: http://www.legislation.gov.uk/ukpga/1988/13/contents
(US inquests -can- involve juries, varying by state, but rarely do.)
For the UK homicide year thing, I have no idea why, but it appears to be true. Quoting:
"Caution is needed when looking at longer-term homicide trend figures, primarily because they are based on the year in which offences are recorded by the police rather than the year in which the incidents took place."
Link: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/116483/hosb0212.pdf
However, I was incorrect about the "when the trial is complete" part, as I missed this section:
"Homicides are often complex and it can take time for cases to pass through the criminal justice system. Due to this, the percentage of homicides recorded in 2010/11 (and, to a lesser extent, those recorded in earlier years) to have concluded at Crown Court is likely to show an increase when the next figures from the Homicide Index are published in 12 months' time."
So if a homicide is recorded, but hasn't passed through the Crown Court yet, it still gets assigned to the recorded year.
Which all sums up to "The UK has a completely different process just for deciding what counts as a homicide." A few particular issues - the US treats criminally negligent deaths -and- manslaughter as homicide (see http://www.fbi.gov/about-us/cjis/ucr/summary_reporting_technical_specification_version_1.0_05-25-2012.pdf ), whereas the UK treats them as manslaughter. Additionally, the US includes justified homicides, whereas apparently the UK has a policy of removing such cases from the record. "Since 1967, homicide figures for England and Wales have been adjusted to exclude any cases which do not result in conviction, or where the person is not prosecuted on grounds of self defence or otherwise" per http://www.publications.parliament.uk/pa/cm199900/cmselect/cmhaff/95/95ap25.htm , which I'm assuming is accurate as their statistics are a -mess-. I'm trying to dig through them now.
Additionally, a lot of their homicides are uncoded. (Something like 50% of their homicide convictions involved deaths which weren't coded in their statistics as homicides, and only got coded as homicides after the conviction.)
As for why it's all this way? I have no idea. It may be they want more favorable statistics. It's also possible that the information they're gathering wasn't originally intended to be used this way. It may simply be that they prefer to code the knowns than the unknowns, whereas the US has a tendency of coding all unnatural deaths which aren't apparently suicide as a homicide. It's certainly true that the US "murder" rate is inflated by unintentional/negligent deaths, manslaughter, and self-defense/justified homicides.