Despite the breathless shock and the pleading rationalizations of TV news anchors, I don't believe that the rioters, looters, and vandals in London over the past week have any responsibility to explain or to morally justify their actions at all. These are angry, frustrated people living under a national and global system that, itself, cannot be morally justified...one that is designed to exploit its citizens economically, indiscriminately murder them in wars based on lies, rob them through slick financial instruments, and systematically loot the world of all of its natural resources. This is a system that is designed and run to destroy lives and destroy the world for the benefit and pleasure of the elite billionaires. How dare they ask the rest of us to morally justify anything.
When the world is, indeed, run by entirely selfish elite bankers, big oil, big defense, big pharma, etc., and no real democracy exists which would give its citizens the opportunity to instrument any real, fundamental change in the system (at least in the US or UK), when the government itself is clearly corrupt and illegitimate, then any action that involves the looting or destruction of property is probably not unjustifiable at all.
If individuals exist who are willing to risk enormous consequences (prison) to go out there and express this anger and frustration, then so be it. When you oppress, sometimes people get fed up and fight back. It is what it is and it should be totally expected.