It looks like the police are getting stretch pretty thin. What would happen if they put in place a mandatory curfew? Like if you are the street after sundown you are arrested. That would be around 8pm'ish or so Guessing on the time because I am in Houston and I know sundown is a lot later there.
They could do that, but this has been going on all day, so it's not so much a 'cover of darkness' thing.
The Police in London have been criticised for years and years for being prejudiced when dealing with ethnic communities, and for being too violent towards protesters. Only two years ago at the G20 summit here, an innocent man was shoved over by police and died.
I suspect that the Police have been holding back to a certain extent because they've been put in an impossible situation. If they react too hard, they will be blamed both for ethnic violence and for brutality. They can't risk being put in that situation again - politicians find it too easy to shift blame to the police if things go wrong. The Police commissioners are not democratically elected (as in the States), so they don't have to answer directly to the public - they're more afraid of politicians.
I expect that they're waiting for the Prime Minister to return - and I assume he'll be the one to tell them to take their response up a notch. UK police have long dealt with crowd violence (especially in Northern Ireland), and they are equipped with rubber bullets/water cannon/CS gas etc. I would expect all these things to be in use later tomorrow.
London is not structured like Houston. It's a vast city made of many interconnecting towns and villages that have grown together over time. The rioting seems to be happening around some of the medium-sized centres in mixed-race areas. The centre of London has been pretty unaffected, and other areas of London such as where I live haven't been affected (I think it's because there are no 'big brand' shops on my local high street, and also the different ethnic mix where I live). There would never be an 'all London' curfew - although there may be some selected curfew on certain streets.