"It is only a knife"
Unfortunately some officers in the UK do not take the knife very seriously until it is to late.
I often hear "It is only a knife".
Very few officers wear knife resistant body armour although it is provided.
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My brother-in-law is a Dr. for a swat team in the Frisco bay area. They are required to watch the vido (of course I forget the name! )which is famous for the 21 foot rule. He's promised to get it to me a number of times but hasn't yet - they take knives very seriously.
Do you have the option of carrying a baton of some sort, perhaps a telescoping type? A few weeks training in elementary Filipino arts and you'll be ready to face a knife safely IF you have time to get it out!
The baton can be (almost) as fast as the knife and yet has the reach advantage, a good choice of tool to face a knife if you don't have a firearm.
"Fear, not compassion, restrains the wicked."