Much appreciate your bringing up the U.K. here. It's is so relevant.
The deadliest mass shooting in U.K. history was the murder of 16 schoolchildren and a teacher in Dunblane, Scotland, and after it happened, the U.K. took steps to get guns off the street. That's vastly different from the situation in the U.S. where nothing happens no matter many shootings we have.
I encourage anyone interested in national differences to read the "Dunblane Massacre" entry in Wikipedia, which gives you a good if brief overview of the British response: