The TV news went further than the BBC online article about the London terror simulation: "innevitable", they said.
I don't disagree that some kind of attack on the UK (probably London) is pretty likely, eventually. I suppose that makes it innevitable.
Essentially, we simply trust that it won't happen. I mean, anyone could do just about anything, anywhere. We just trust that no one is motivated enough to actually carry it out. If someone, or more likely a group, were sufficiently motivated to get a nasty chemical, or nuclear material, they could carry out their attack anytime, and more or less anywhere. There is no way that such an attack could be physically detected, an thus we are naturally vulnerable. Further more, our vulnerability is actually designed into our society. We expect the freedom to move without being accountable for it, and so we have to accept that an attacker may abuse this ability against us.
I struggle with this. It seems a problem without a solution. I mean, you can reduce the likelihood of sufficiently motivated groups actually existing. You can also observe and investigate suspected future-perpetrators, but ultimately, the possibity always exists. Clearly, tightening society in order to make it more difficult to actually carry out an attack is to make it happen (reducing your freedom is essentially what the attackers aim to do, so doing it yourself isn't productive).
Around the simulation in London, it has been said that too few people know how to react to an attack. Apparently every American police officer knows what to do. It's also true that if everyone knew how to survive, more of us would, well, survive.
So, it seems the only real solution to the problem is education and preparation. Who knows, maybe that education might actually reduce the likelihood in the first place.