Bristol only had 300 officers on duty. They are too small a force to have the resources that the Met can call on. In the circumstances all they could do was defend the Police station and prevent it being taken and burnt as happened in Minneapolis post George Floyd. Other forces sent aid but without pre-planning they weren't of sufficient number to enable a more robust response. The logistics in effecting just one arrest would have severely depleted the cordon around the Police station to a point where acute danger was caused to those remaining.
I append a quote from a Police site that I am on that, whilst it is 'hearsay', I have no reason to doubt:
"I’ve been retired for 2 years now but they were my friends up there last night. What you won’t see in the press is that an officer got dragged into the crowd and badly beaten and they were setting fire to the crew buses with officers sat on them! My blood is boiling."
It would appear that there were those in the mob whose intent was literally to kill the bill.