I think you are correct in that, at a certain point there is a tipping point and people will start to change their behavior either consciously or sub-consciously.
This is a worrying new tactic in that the lone wolf attack is easier to subdue once in attack mode, a van with people jumping out the back means you could in theory have a lot more attackers delivered at a scene (or multiple) scenes with attackers spreading out in all directions possibly encountering people running from other attack locations. So now it's threat level down, looking for weird loners, but now I suspect it will be threat level up again and we are looking for a group of pricks with a van and fake vest making skills. Not a stable outlook for anyone at the moment where counter-terrorism is concerned.
Also, the fact that it it more than one attacker means there is a lot of people who know these terrorists. 3 people have a lot of interactions with a lot of institution's (Bank's, schools and places of worship to name but a few) and within their own communities so it was again missed by everyone. I suppose if, for example they are brother and / or cousins this is a way to keep it relatively under the radar but this will no doubt inspire similar multi-attacker attacks unfortunately.
The wearing of a fake vest means death is almost guaranteed as well which adds another dynamic to it all as they are more useful alive for a short time to the authorities I would guess.
Given that there is a general election in a few days I see no other way of protecting the Polling Stations (All) than using Army resources. That is going to be a logistical nightmare to even make it come across as safe, if it goes ahead and something happens then that will stop the election and cause people to think about going again maybe ?