There are doubts about how viable water cannon would be in London, where riots broke out almost simultaneously in many different areasin the last 48 hours, quite apart from elsewhere in the country. Obtaining water supplies may be difficult in riots, and there is the risk of internal injuries to demonstrators.
How about the police just call the rioters "Poo-Poo Head" through bullhorns and see how that works?
Of course, then the government would have to sponsor self-esteem workshops to repair any psychological damage that ensued.
I’d say they’re a few days late in using real cannons. Like the 20mm type.