Red Rat, Post 12, stated in part:
Generally UK Police forces prefer to contain public order situations, preventing damage to property and minimising risk to life....then with things have died down they go in days and weeks later and arrest those involved using evidence they have gathered during the riots.
The police (PSNI) have started their arrests for the violent disorder two weeks ago:http://www.bbc.co.uk/news/uk-northern-ireland-10791364

Referring to a juvenile arrested and at court opposing bail:
A detective sergeant who said he could connect him with the offences told the court: This individual I would say, having examined CCTV footage, would be one of the most prominent individuals within that riot. From the very start right through until the early hours of the morning his activities were wide-ranging from attacking police officers with poles, pipes and other materials to the hijacking of the vehicle which was set alight.