That would mean that the U.K judiciary system doesn't actually understand what it means to actually disable someone as a threat. If that person can lift their arm and look down the sights of their weapon, they are still a threat to your life. If you, or anyone, thinks that that is excessive force, then they don't know anything about the human body or about how bullet trauma works.
You can be shot dozens of times and still live; if you can make it to a trauma center within a certain amount of time. As long as certain organs are not damaged, you can theoretically be patched up. People have even survived grazes to their heart, although rare, but still possible, you could be shot in the head, the bullet could damage part of your brain but you still live. Although generally, direct shots in these places should put you down for good, the moment it happens.
The only way to disable someone without necessarily killing them is by shooting the spinal column above the arms but below the neck, and somehow avoiding the major arteries right next to it. Completely unrealistic, the fastest and quickest way to keep yourself from getting killed by someone else with a firearm is by shooting them center of mass as many times as you can. Even at that point, if you're aim is off, that person might still be dangerous even after you've loaded an entire magazine into them. Not everyone behaves the same way when they are shot, some people curl up in a ball, some people only get more angry and more psychopathic and dangerous if you do not neutralize their ability to hurt you. To immediately neutralize a threat, the choice that that man made in that video is completely necessary.