Security expert Bruce Schneier links to an MIT report saying that most terrorism does not achieve the stated goals of the terrorist group because the attacks are often on civilian targets, not military ones. Such attacks are counter-productive because the population assumes the group is out to kill them all, not just reach a stated objective. Thus, the target country then becomes convinced that negotiation is pointless and the blowback becomes ferocious.
As for the recent UK attacks, he says
First London and then Glasgow. Who are these idiots? Is there a Special Olympics for terrorists going on in the UK this week?