That's horrifying, but the thing is, very few criminals in the UK have guns, and even fewer have something they can conceal under a jacket. We do have armed police in the UK, and the way it works is that officers do a firearms certification, shooting hundreds if not thousands of rounds during the initial training alone. They then either work from an ARV, similar in capability to a US swat team, or go out on patrol carrying a sidearm, but the principles on which they would actually deploy their firearm are different to US police. In the US, lethal force being justified is a free pass for lethal force being used, whereas in the UK, it's only used if it's the only feasible option. The guy in that video would likely have been tasered immediately if there was evidence to suggest he would be carrying, and then cuffed and searched. If it was a preplanned operation then firearms officers (and we're talking a lot of officers, the equivalent of an entire SWAT team) would have pulled up in marked cars, and pointed a lot of rather intimidating looking longarms at him. Apparently current strategy is that there's a taser officer with a shield meaning that within a certain range a suspect can be tasered rather than shot even if they go for a gun.
And speaking about the above scenario, I'm skeptical as to whether anyone would've been able to draw their sidearm or even sight an already deployed gun before that suspect started shooting. If there was the slightest evidence to suggest he was going to have a firearm I guarantee in the UK he wouldn't have been given time to realize he was being arrested, let alone start shooting. The fact that a single officer was deployed, armed or not, to this guy is the cause of his death, and not him not being better equipped.
Believe me, I'm pro police carrying guns. There are plenty of forces in mainland europe who arm all of their officers with sidearms and SMG's and still have a breathtakingly low amount of shootings, but the situation in the states where firearms are routinely unholestered and pointed at suspects who aren't armed or anywhere near dangerous enough to need a gun pointed at them is not at all compatible with policing by consent.