Unfortunately, your "statistic" to "shut people ... up for good" isn't actually true.
Suicide and Murder Rates for the US and Great Britain are about the same
According to the United Nations (warning, .xls file), the intentional homicide rate in Great Britain (the UK collects different data for England and Wales, Scotland, and Northern Ireland, so I've combined the E+W and Scotland data to get a GB one) in 2011 was 1.1 per 100,000. In the United States it was 4.7. The suicide rates are similar, but the intentional homicide rates are way off; over four times as high.
That said, according to the FBI about 69% of homicides in the US in 2012 involved the use of firearms. So ignoring all firearm-related homicides, the US's homicide rate is about 1.4 per 100,000, so still higher than Great Britain's.
But none of this means anything on its own, as far as policy implications go. Working out whether bans on certain classes of firearms are necessary and/or proportionate is a very complex task, and a couple of statistics are hardly conclusive.
Your "statistic" was still wrong, though.
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On the murder of Lee Rigby, there are a few subtleties you may have missed. First the assailants killed him by hitting him with a car, and then stabbing him with knives (apparently unable to decapitate him) before anyone could react, even if they were armed - it's unclear if there were even people nearby at the time (one of the first 'witnesses' got involved after thinking it was an accident and trying to give the victim first aid). The assailants had a revolver, so it wasn't just knives; it is possible that if they had anticipated resistance from someone with a firearm, they would have shot him (rather than stabbed) and may have shot others nearby.
The suggestion that anyone would have "kicked the shit" out of them is rather ludicrous in any event. As it happened, after their attack the suspects waited calmly for the police to arrive (talking to passers-by); when the police did approach, they charged them ineffectually, were shot, and taken into custody.
I fail to see how widespread access to firearms would have made the situation any better, or how passers-by beating them up would have furthered the interests of justice.