It's not native Britons rioting, largely it is young members of immigrant communities. However, given the apparent youth of the rioters I doubt racial politics is a significant issue here. It appears to indeed be rioting for the sake of rioting.
This is not 'immigrant communities', this is people who were born here.
NB - I'm not normally a stickler for sourcing, but I feel a statement as controversial as this requires one.
What has been reported is that it's youth from poor neighborhoods. Considering the population composition of those neighborhoods, I'm going to guess that it's probable the groups are composed of a majority of people from those immigrant communities. It still doesn't mean it's a immigrant riot. I bet poor non-immigrant (as in their family always been there) are also participating.