The UK also defines "crime" differently than the US. Many murders are downgraded to manslaughter over there, since it is easier to get a conviction. Anyone under 15 (or so) isn't even included in the statistics! That is the largest criminal demographic in the UK!
In addition, the stats only track convictions, whereas US stats track anyone even accused of the crime... it does not necessarily follow them all the way through to conviction. So, someone in the US could be convicted of murder, but later be acquitted, yet still have that number go into the official FBI report for the year.
Even if one does look at the stats from the Home Office, tell him to take a look at a simple line graph of historical stats. He'll find that after 1995 (total gun ban), the murder rate actually stayed the same, even going up some. What? How did that happen? Shouldn't murders have DROPPED? Instead, they went up? Only very recently has it come down a bit, but it's still within their historical average. How do things stay the same? Simple, gun control doesn't, never has, and never will work for reducing crime.