One of the really important things is to keep a sense of proportion. The last time I checked, Greater London had a population of about 12 million people. Given that population, the city should see in excess of 100,000 deaths a year from all causes, or about 300 per day. The deaths caused by the occasional terrorist attack are a negligible fraction of the total deaths. Terrorism is a nuisance, not a major threat.
Even the 3,000 deaths caused by 9/11 were not terribly significant; the "worst terrorist attack ever" killed about the same number of Americans as are shot to death each month by other Americans, and caused property damage (around $50 billion) equal to a fraction of a percent of American GDP. The response to 9/11, on the other hand, has killed at least hundreds of thousands, maybe millions, of people and cost the US literally trillions of dollars.