From his Wikipedia page

Hitchcock was too young to enlist when the First World War started in July 1914, and when he reached the required age of 18 in 1917, he received a C3 classification ("free from serious organic disease, able to stand service conditions in garrisons at home ... only suitable for sedentary work").[37] He joined a cadet regiment of the Royal Engineers and took part in theoretical briefings, weekend drills and exercises. John Russell Taylor wrote that, in one session of practical exercises in Hyde Park, Hitchcock was required to wear puttees. He could never master wrapping them around his legs, and they repeatedly fell down around his ankles.

So, none.