This tale is of course fiction, well most of it is. As has been pointed out to me by people who know about these things, there is nothing in this book which serious terrorists could not do. Maybe not quite on the scale suggested here, but who knows?
On the other hand, much of what was available to the security forces to help counter the attacks, either does not exist or they are not allowed to use. The levels of political and judicial interference are real enough, in real life these simply have to be lived with.
Many of the problems in real life counter terrorism arise from the need of a given agency to score 'brownie points' to gain funding. This is a home grown political problem which is rapidly becoming endemic and can only lead to vital information being overlooked, the result of which may not be dead birds at the Tower of London but many dead people.
Not the numbers of dead in this story, hopefully, but if it happens to be you or your family, then the numbers don't matter. Sleep well.