Thats because in the UK, the police are unarmed. If they step out of line they get beaten back into line. In the US, some one could be laying face down on the ground handcuffed and get shot in the back for "resisting".
It's true that, especially at protests things don't always go the right way, the problem is though, the murderous psychopaths out there are still going to get you one way or the other and the police will be even less capable of responding to it
So you have to make sure that you restrict the murderous psychopaths' access to guns as well.