I wouldn't like to be making the political call this morning. Army on the streets will look like they've lost control, and will make the Police look like they can't handle the situation.
But - they clearly can't handle the situation. And they can't just simply let these scummers smash and loot vast areas of London. They need to do something, but whatever they do they'll be damned.
The big problem now is, the establishment have let the rioters/looters know that they can't handle the situation, and that they have a green light to do whatever they want. That's the reason it spread so wide last night - even spreading to other cities.
I'd love to see the authorities crack down hard, really hard. But would that make things worse, I'm not sure. What if the violence escalates, there are a lot of illegal guns in London - thankfully these haven't been turned on the Police/public yet...