The pistol ban came about because the Cullen report coincided with a General Election and the promise to ban all handguns was a cheap throw away election promise by Tony Blair. It cost billions, people lost their businesses, and their homes. Like other posters have already said, it achieved nothing.
You have to remember there were some shadowy organisations like the Snowdrop Campaign pushing for a ban. The Snowdrop Campaign was supposedly a group of outraged mothers. If you believe that you will believe anything. They were funded and organised.They collected millions in donations and then disappeared without a trace. Never published any accounts.