The riots spread in London because neighbouring gangs thought, "oh, look at all the crazy stuff that gang in Tottenham is doing! That looks like fun! Also, we don't want to be outdone, so lets make ours even more fun!" That kicked off riots elswhere in London. Other gangs in other cities saw the same thing, and thought the same thing, but the 2nd impoetus of "not being outdone" was much less significant, because obviously gang rivalries are stronger between Tottenham and Hackney than between Tottenham and Manchester. I don't even know if gangs in London are aware of the existence of Dundee, so I doubt rivalries would have been at all significant there. It was more a matter of luck as to whether an individual gang decided to make noise elsewhere in the country.
The looting came after the gang-instigated rioting. As gangs smashed shop windows, the looting started; herd mentality took care of the rest, turning ordinary people into opportunistic looters.
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I've said this before, the question isn't "how do we prevent riots?" but "how do we deal with criminality?". There may be socioeconomic or cultural causes for the prevalence of gangs and criminals in London and other cities, but ultimately talking about the riots is just a useless distraction. The riots and looting were merely the most visible symptom.