You have to be kidding! Our time-travelling historian can provide a lot of very useful advice, but he can't shift the entire psychology of a nation and government (knowledge of his existence is restricted to only a handful of key people, for fairly obvious reasons). The UK was unprepared - both physically and psychologically - to go to war with anyone any earlier than they did.
Yes, much of that is included in the novel, but the technical limitations of the time have to be borne in mind. Electronic computers and night vision equipment would almost certainly not be possible - our man is an historian, not an electronic engineer, so would not know the details of how they are made.
Yep, all in the novel, plus a lot more - except that as well as starting the development of a Centurion-type 40-ton tank, they would first develop a smaller and much cheaper 20-ton AFV family to build up the numbers quickly. Money for developing and building new equipment would still be very short and little could be done about that, so trying to be too ambitious too soon would be a mistake.
One of our historian's key concerns is to follow the medical mantra: "first, do no harm". A worst-case scenario would be for him to say "don't build that, build this super-duper device instead", only to find that the super-duper device runs into a heap of unforeseen problems, arrives years late if at all, and results in the UK being worse off than historically. So developments need to be taken in small and careful steps, with solid back-ups available in case more advanced schemes fail.
