I'm sure Orange or Carrie have their own answer, but as far as I knew there haven't been any riots since 2011.
They were started ostensibly because of the death of
Mark Duggan who was shot by police when the tried to arrest him. Apparently gangsters have a lot of respec' so they started rioting in Tottenham in reaction to this.
From there it spread into a giant ball of fuck-whittery. There was no purpose, no political agenda behind them, it was simply violence and criminality for it's own sake. Several cities saw copycat action as well. For me the most telling image wasn't the century-old carpet warehouse burning down, or the police being urged to use water cannons, though they were all in Northern Ireland. To me it was the picture of a youth leaving a Tesco with a pilfered bag of Tesco Value Basmati Rice, worth about £2. It was really that pointless and destructive.