The police are completly out numbered, they have done nothing because they are unable too. It may well fizzle out, but what if it doesn't?
Sending armed troops would not only be dangerous for bystanders, but also political suicide for whoever gives the order. Trying to wield military power like that on the streets of London would instantly turn the people against the government. The Brits have long held a deep suspicion of standing armies in the pocket of the government.
The most bizarre thing about this though is how organised the rioters seem to be, especially when they seem to be stupid hoodie-types. Doesn't seem like we're dealing with Che Guevara here...