...there's no reason to presume he would have attacked in the next several minutes.
The video of knife-wielding perp advancing towards the cops while shouting obscenities tends to undermine your assertion.
Someone linked to a UK/taser
advertisement, but even there we read the following:
"Certainly a Taser is an option that’s available to the officers,” said Dotson. “But Tasers aren’t 100 percent. So you’ve got an individual with a knife who’s moving towards you, not listening to any verbal commands, continues, says, ‘shoot me now, kill me now.’ Tasers aren’t 100 percent. if that Taser misses, that [individual] continues on and hurts an officer... In a lethal situation, they used lethal force."
I concur with the immediately preceding assessment, and really don't care how the UK handles their business. After all, they keep their firearms in their trunks, if not locked up back at the station, where they're nice and "safe," incapable of harming anyone...
...or being used on the job as per their design.
The whole reason I included the transcript along with the video is to demonstrate precisely how little time these officers had to respond, much less coordinate how they were going to deal with the situation.
Having said that, if you have a bunch of officers on site dealing with a lunatic who is outraged but not harming anyone, then by all means use a taser.
That wasn't what was happening, here..
What happened is the perp charged the cops while baring a deadly weapon within 15 seconds of their arrival.