London...

Terrorists are a classic contradiction, the successful practitioners all adapt seamlessly to the very societies they are so desperate to disrupt & destroy.  They can identify "soft targets" with malignantly shrewd insight and secure such weaponry as said society makes available to them. That and they have the secrecy mania and advantage of surprise, and that in turn works best of all if this is a one-off lone wolf style attack.  If something like today's tragedy had gone down in the USA the perp would've had easily some large magazine'd carbine or a pistol with a similar capability....again its a question of the weapons easiest to procure and this time it was a hunting knife and a fast car.
Attacks like this are gonna become the norm, low tech one off murderous actors, largely trained on line in a sort of "homicidal MOOC", mass action on hard targets seems largely denied these people so the action gets automatically exported to the streets where determined self destructive recruits can rack up a few casualties and seize the headlines.
In some ways its a confession of sheer impotence, a well programmed hidden asset largely wasted in bloodthirsty street theatre...but I'm sure the news media will see it all very differently.

My Heart Goes Out to the Victims...As Always.