What is the author's tone in A Man Called Intrepid: The Secret War by William Stevenson?

The tone of the book is obviously pro-British and factual. In spite of this, the material is presented in an objective manner as Stevenson relates the details of the various incidents and describes how the decisions were made. The British protected their source of information, the Enigma machine, that they had at Bletchley, even though this meant the loss of thousands of lives.