Logical fallacy
OC-Trainer, you are arguing from some logical fallacies, and you are also ignoring some facts.
Your assertion is:
Armed citizens haven't stopped the mass murders in (your chosen database), therefore, armed citizens cannot stop mass murder.
This is the argument from antecedent fallacy. (Note: use of a logical fallacy doesn't automatically negate the conclusion - the conclusion can still be correct, it's just a sub-par way to arrive at the conclusion.)
For example, I gave my son 160 algebraic equations, and he didn't solve a single one. Therefore my son will never solve algebraic equations. (This argument omits the facts that he is only four years old and hasn't taken algebra, however.)
What would have happened if there were more armed citizens? Who knows? There is no way to know for certain. Also, as others have said, an attack stopped early doesn't even make it into the database in the first place. This is like arguing the Pittsburg Steelers are a great team by looking at a database of their winning seasons (and not any of the losing seasons).
Or, I studied a database with 160 house fires resulting in a total structural loss, and concluded fire extinguishers are useless.
"They use special detonators that still activate the bomb once shot."
How many examples can you cite where they have actually used special detonators? It's actually rather rare, in fact. (And arguing things are so rare that they don't matter - it's kinda your thing)
Heck, there are certainly examples where the suicide vests weren't even real, so yes shooting someone with a fake suicide vest would effectively stop the attack.
They couldn't be bothered to even make a real suicide vest, much less a deadman's switch. "Oh how could they take the chance of blowing everybody up". Well, given there are fake suicide vests, and few examples of actual deadman triggers on real suicide vests, and the fact that someone with a real suicide vest is likely intent on detonating it, I'd say it's a reasonable course of action to shoot such a terrorist in the act of killing victims. (Besides, do the police have special "suicide vest disarming" bullets?)
Police might shoot the "good guy with a gun". How many examples can you cite where that has actually happened?
Will there be a clear case of an armed citizen stopping a terrorist attack? Probably not.
Certainly not in Britain, so that police chief can sleep soundly at night knowing he, at least, won't be ever be wrong about that, since Britons would never even get the chance.
If Britain suddenly reversed course and allowed their citizens the right to effective self-defense, would one of them stop a terrorist attack in progress? Probably not, but I wouldn't say "never".
There are scenarios where escape wasn't a possibility for some victims, due to the terrain (walls) or disability, so "run and/or hide" doesn't save everyone. There were scenarios where a citizen armed with a gun could well have stopped the attack.
Overall, the odds of someone stopping a terrorist attack are indeed quite low.
That's still a lousy argument for denying the populace the means of effective self defense. It's disgusting, in fact.
Now, be on your merry way to declaring yourself right (and presumably deriving great pleasure from it). Score one for keeping Brits disarmed!
I meant "there."