Two things:
1. About the only thing that can be done is to root out the instigators of the disorder and deal with them.
Controlling a mob without force is almost impossible, unfortunately, as you say the civilian authorities have their hands tied, and short of having counsel and judiciary on site to process those who are intent on rioting and looting, there is really nothing they can do but watch.
Martial Law would ultimately stop the behaviour, but wouldn't be pretty. And would be seen as an act of oppression.
Eventually normal people are going to have enough of the mindless behaviour. And will expect something visible and decisive to be done.
2. 14% of the UK population is non white, in the US, this is nearer 28%, but at 12%, there are fewer blacks than Hispanic/Latino (14%).
Racial minorities should not be victimised by the fact that any representative system will always under represent them. This is pretty clear cut here as the census data groups are clearly defined with only 8 ethnic groups...
https://en.wikipedia.org/wiki/Demography_of_the_United_KingdomIn the States, there are 20, which serves only to further minoritise non whites...
https://en.wikipedia.org/wiki/Demographics_of_the_United_StatesPerhaps, instead of identifying everyone by colour or ethnicity, we should move to identifying people by residency... ie, if you live in the UK, you are British rather than black or white. If you live in the US, then you are American.
As a citizen/resident you agree to be bound by the law of that land, and in return, the law, and its government and officers, respect the right to honour the ethnic and religious traditions of the individuals and communities with absolutely no prejudice. This could also be applied to sexuality, although traditionalists won't be happy.
Can it really be that hard?
On a side note, speaking as a white heterosexual middle class male, why do i feel like a minority in all of this?