It is interesting to note in this context that in the last ten years what might be called a “zero tolerance” attitude to terrorist risk in Great Britain has become more widespread. While it has always been the case that the authorities have made every effort to prevent terrorist attacks, it used to be accepted as part of everyday life that sometimes the terrorists would get lucky and there would be an attack. In recent years we appear increasingly to have imported from the American media the assumption that terrorism is 100% preventable and any incident that is not prevented is seen as a culpable government failure. This is a nonsensical way to consider terrorist risk and only plays into the hands of the terrorists themselves. Risk can be managed and reduced but it cannot realistically be abolished and if we delude ourselves that it can we are setting ourselves up for a nasty disappointment.
Those who are interested in such things might find the Intelligence and Security Committee report Could 7/7 Have been Prevented (PDF) gives greater insight into the Security Service’s focus and capabilities.