> The UK has had several massacres where in each case over a dozen were killed with no apparent motive (Dunblane in 1996, Cumbira in 2010), and there have been several smaller scale incidents along similar lines.

This is disingenuous. There have not been 'several cases where over a dozen were killed, and several smaller scale incidents'. There have been exactly three in recent UK history: Dunblane (16 schoolchildren killed, and has to be the worst criminal act in modern UK history), Cumbria (12 adults in a lone gunman killing spree), and the Hungerford Massacre in 1987 (16 murdered by lone gunman).

Going further back, there is the Shepherd's Bush murders in 1966 (3 policemen and 2 members of the public shot dead) http://en.wikipedia.org/wiki/Shepherd%27s_Bush_murders as a possible example of 'several smaller scale incidents along similar lines.' But this is hardly how you phrased it.

Now look at the United States list as a comparison: http://en.wikipedia.org/wiki/List_of_massacres_in_the_United...