Had a very quick look around. Based on
2008 figures:
A prisoner costs around £40,000 per year to keep in prison. You want to keep the rioters locked up for 5 years so £200,000 per prisoner. So far, there have been 750 arrests in London alone. Even with a 50% conviction rate, that totals to £75,000,000 to keep those in prison for 5 years.
In this climate, where are we going to take that money from? Not to mention the cost of building all the new prison spaces. The article mentions £119,000 per prison space. So that's another £44,625,000 if we need to build them all.
All that and not one of the prisoners has paid any debt to society. At least with community service work they are doing something to repair the damage they have caused.