Your Brits argument is based upon a logical fallacy that assumes that every gun homicide would be prevented if guns were not present. This is a false assumption and is in fact totally illogical as it ignores the issue of substitution. Murder (a prime subset of all homicides) is, by definition, an intentional act. When one tool is unavailable, generally another tool will be employed to accomplish the intended result. However, this is not a one to one relationship, as other factors come into play. Firstly, one of the advantages of firearms is their comparative lethality compared to other methods. You can expect more victims to survive a club attack as opposed to a gun attack. Thus, everything else being equal, you can guess that there would be more "succesful" homicides in countries that have more guns in private hands. This is offset by the deterance factor as some perps will decide not to attack in the first place if they fear their intended victim may be armed. Thus, in countries with strict gun control, you might find a greater number of attempted homicides... with the net result in overall homicides being almost a wash.
This is in fact the experience of Countries which have introduced strict gun control. One need only take a look at the Brits themselves. Modern gun control in the UK had its inception in the aftermath of WWI. Although sold to the public as a measure of crime control, the over riding reason for the passage was the revolution in Russia. The Lords and Lordettes were deathly afraid of a repeat in the UK and thus was born the Firearms Act of 1920. So what was the homicide rate in the UK prior to the passage of the Firearms Act? Take a look at the historical trend in the UK from 1857 to 1993 available here:
http://www.hoboes.com/pub/Firearms/Data/England/English homicide rates/
The most current rate (2004 UN stats) for England and Wales is 1.6/100,00 (Scotland is 2.6 and Northern Ireland is 2.4) ... so almost double the rates that prevailed in the 15 year period prior to the Firearms Act of 1920.
http://www.unodc.org/documents/data-and-analysis/IHS-rates-05012009.pdf
Your Brit makes a cross cultural statistical argument based solely upon homicide rates between the US and the UK and finding a correlation, he assumes causation. If gun availability were truly a causative factor, then a comparison of countries other than the US should provide similar results. In fact it does not. See, Don B. Kates & Gary Mauser,
Would Banning Firearms Reduce Murder and Suicide: A Review of International Evidence, 30 HARVARD JOURNAL OF LAW & PUBLIC POLICY 651 (2007) available for download here:
http://papers.ssrn.com/sol3/papers.cfm?abstract_id=998893
Another issue your Brit does not even consider is whether gun control laws could possibly be effective in the USA. For a powerful argument that they would not be, see, Nicholas J. Johnson,
IMAGINING GUN CONTROL IN AMERICA:UNDERSTANDING THE REMAINDER PROBLEM, 48 Wake Forest Law Review 837 (2009) available here:
http://lawreview.law.wfu.edu/documents/issue.43.837.pdf
