There is alot of planning that goes in to all of this. My friend works for gchq and she was part of a team that indentified places all around the country liable to be attacked. They study the areas look at problem points and areas that don't have much CCTV. Many many reports are created and action taken. I imagine this story is a tiny tiny part of what has been done. And merely the tip of the iceberg. And one of the very few things that are made public.