I don't think they should let them get away with it, but I think they should run video cameras and put protesters responsible in jail for destruction of property. Also, I think something as simple as turning a hose on the protesters can get them in line. They're all fired up and angry... so cool them down and they can come to their senses. I think there's also a type of gas that can be used that stings and burns, but doesn't actually cause any long-term harm.
Having a right to protest is not the same as having a right to fire things at people or destroy property. At that point it's not a protest, but a riot.
Of course, I would support the right of police to use shoot people who have guns in the US, but that's because once the criminal has a gun, your options are limited and it may be a kill-or-be-killed situation. Good thing about the UK is that that's far less common.