How ever many police you have, they will still be outnumbered by the rioters. Also, it takes more than one policeman to arrest someone who is rioting, mainly because they are running on adrenaline and are going ape ****. They can't be more aggressive, because if one tiny thing goes wrong, like someone falls over and breaks their arm, the police will get in ****, policemen will lose their jobs, all because people cant understand that the police are there for a reason, they are to protect people that need protecting. Rioters should be battered by the police, or army, because they are putting innocent peoples lives in danger. Not only that, but they are completely ruining areas where people live/work, which is going to have a much longer lasting effect on regular people than someone nicking 50 dvd's from HMV.
At the end of the day, if people see this going on for too long without major repercussions, they will think they can get away with it, we need to crack down on it, and hard.