You could've armed every single British citizen within a 10 block radius of that incident before it occurred, and I just don't see how any one of them could've responded fast enough to prevent than police officer from being shot. However, they may have gunned down the murderer afterwards, which would've been at least satisfying, except if any stray bullets killed innocent bystanders.
So what will happen to this guy? Do British prisons have cable TV and weight training equipment, and free educational benefits too?