It's funny that it was the last protest that got the response it did. The previous ones, including one where a bus was burned was just some light hearted shenanigans, that was totally acceptable.
It's really weird what happens when the protest is planned a day in advance (Southport) or takes place in probably the most heavily policed part of the country with the biggest police force, vs something that happened within a couple of hours outside of the capitol.
Apparently the idiots were throwing flares etc at the Downing Street protest, which is particularly stupid given there are a lot of armed officers and the Prime Minister etc are there, by some of the comments about the airport the police should have opened fire.
It's also weird how the police response varies depending on the ongoing risk to the general public.
It's almost like the police will treat a mob known for it's violence in the past, that have been planning it for a day (giving the police plenty of time to get ready themselves) and are there to go after individuals/a local group with rather more force than a group that might be doing some property damage but isn't an active threat to other members of the general public at that time.
And note I'm not excusing either group, what I am saying is that the police react different based on how much time they have, the level of violence, the current or expected risk to the public, and what they might know about specific major players in the groups. A lack of arrests in front of cameras on the day means very little with how much information the police can get on who was in an area.
Going back to various other protests that turned violent, the police response on the first day if it was unexpected has always tended to be low and more about containing if possible, because it takes time to organise hundreds of police officers, get them decked out in riot gear, loaded onto the busses and transported potentially hours away from their normal operating areas. Back during the Duggan riots IIRC it took the better part of a day just to get officers in place to start getting things under control*, and whilst the police may not have made many arrests early on, they were still arresting people something like 18 months later.
*And in the process, from memory they stripped the coverage from a large part of the country, even with cancelling all leave etc.