I’m planning a story with an English character who joined the army during WW1 (because he didn’t want to be considered cowardly) but eventually became disillusioned with war and with the British Empire. He was born physically disabled but managed to conceal the disability in order to enlist. Are there any disabilities for which this would be plausible?
No. It would be difficult to hide any serious disability during the recruitment medical examination. In 1914, the sheer volume of recruits meant that examinations were fairly cursory, but, anything significant would have gotten washed out. Also, he wouldn’t be alone in that respect, somewhere between 40% and 60% of volunteers were turned away for being medically unfit.
There two major exceptions, that were sometimes, “overlooked,” by the recruiters.
The first was height, a British Soldier was required to be at least 5’3″ (later revised up to 5’6″ to reduce the number of recruits that were being processed), though this was not always strictly adhered to.
The second was age, the British military required recruits to be 19 or older, though estimates put the number of underage British soldiers who served in World War I at around a 250,000.
It is important to understand, “fear of being viewed as a coward,” was not a leading cause to join up. Certainly not for someone who had a disability that they would need to work to conceal. Peer pressure was a factor among underage recruits. For adults the leading factors were patriotic impulses, or an opportunity to adventure.
Worth remembering, World War I was a significant turning point for the perception of warfare in Europe. It was brutal, and destructive on an incomprehensible scale. Twenty million people died from 1914-1918. Another twenty million were seriously injured.
That last part is important for reference. There were a lot of soldiers that went to war with the idea that it would be a grand adventure. They swore to protect king and country, but returned horrifically maimed, after receiving front row seats to industrialized warfare’s opening act.
War has a long history of taking young, healthy individuals and returning them in much less intact conditions. Even if your character left for war in good shape, it’s entirely plausible they wouldn’t return that way.