Believe me, what we are dealing with now is vastly different. Terrorism is terrorism end of but the idealogy behind these groups are diametrically different.
I don't doubt there were differences in ideology as well as method. The most recent terrorism has been a succession of global campaigns and (while Irish terrorists did occasionally go for civilian kills) has been unusual in going almost exclusively for soft targets. In killing, though, it's indisputably the case that the Irish wave of terrorism had a very high kill rate early on.
It looks to me as if this was reduced greatly as intelligence and security improved (and perhaps as those running the campaigns recognised that killing civilians ran counter to their aims and reduced their grassroots support).
We're more tolerant of anti-terrorist security activities in the UK than we have ever been, and London in particular is somewhat infamous for the high degree of camera surveillance. The fact that camera surveillance isn't very effective ought to be quite evident. But awareness of terrorism as a problem tends to lead to increased awareness and vigilance, and penetration of terrorist cells further reduces the ability of cells to plan attacks.
Recently IS has been reduced to attempting to inspire freelance attacks, employing ad hoc methods such as vehicles and knives. Such attacks are effective at first because of surprise, and because the disruption of communication networks is ineffective when there is no network. We should expect them to reduce in incidence over time because they rely on the existence of highly radicalised individuals not yet being watched closely. The security services can just pay more attention and close the gap. Likewise, event organisers can make their plans in coordination with the police in order to reduce the concentration of easy targets where this is possible, and commit more resources to on-site surveillance.
