I have to point out that most of the US presidents who got assassinated didn't have proper bodyguards. Only at some point the US government realized that maybe the president is worth protecting but then they just decided that whichever agency happened to be the closest gets the job. Therefore the folks who had the job of finding counterfeit money became bodyguards for the president.
...And several decades and assassinations later (soon after JFK's death in fact) people actually realized that handing the bodyguards some firepower also might be useful. Up to that they had snub-nosed .38s and that's about it.
And now that the presidents are under the guard of men and women with assault rifles, smgs, and high-powered pistols they have had much higher odds of survival.
Meanwhile in Britain despite having more firepower than the average city in UK the security detail are unable to stop one dude with a knife.