https://www.thesun.co.uk/news/4470415/parsons-green-terror-attack-hunt-isis-bomb-arrest-raid-surreyLondon train bombing. These terrorist attacks are beginning (??) to resemble the Hollywood trend toward making remakes of remakes of remakes of successful movies. How many times has the story of Robin Hood or The Lone Ranger, or maybe Tarzan, been "reimagined" on the big screen?
So London has another terrorist attack. Following the attack, they flood the streets with armed cops and raise the threat level to "high." This will last for a week or two, maybe a month, and then people will tire of seeing cops on the streets with guns, the cops will tire of traipsing around with all that combat gear on, the politicians and administrators will tire of paying for all those armed cops to be on the streets ... so the police "protection" will taper off, things will revert to the way they were before ... and then they'll have another terrorist attack of some kind.
I expect the same here in the U.S. What makes it more likely in London is the extremely high percentage of the population that's Muslim.